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Fig. 26. —Wood-ant (Formica rufa), Europe (natural size). a The winged male. bThe wingless neuter or worker.

ff Mandibles. h Max

Order IV.-DIPTERA (Two-winged), generally called Flies.

Characters: Insects having only two wings, but generally with a small pair of organs named halteres or balancers, in place of the inferior wings; mouth suctorial. Figs. 27 to 31.

Fig. 27.-Hoverer Fly (Syrphus). a a Antennae. b b Eyes, cc Halteres or balancers. dd Lobuli or lobes of the wing.

Fig. 27a.-Head and thorax of dipterous insect. a a Antennae. b b Eyes. c Thorax. d Abdomen. ee Wings. f k Alulae or scales (abortive wings) protecting the balancers. gg Lobuli of the wings.

Fig. 28. –Antennae of Diptera. a Verticillate or whorled. b Lobulated. c Plumate.

Fig. 29.—Metamorphoses of the Gnat. A Boat of gnats' eggs. B Eggs magnified. a Eggs attached. b Separate ‘ī; with lid open for escape of the larva. c The larva. c 1 Larva magnified. e Respiratory tube. f Anal fins. 7 g Antennae. D Gnat escaping from the pupa-case. E Perfect insect. a a Antennae. b Rostrum.

Fig. 29a.—Sucker of Gnat (greatly magnified). a Sucker in its sheath. b Part of the sheath removed to show the piercing instrument.

Fig. 30.-Horse-fly, eggs, and larvae. a Eggs attached to a horse's hair. b Larvae or bots adhering by their mouths to the inside coat of the stomach.

Fig. 31.-Mouth of Gad-fly (magnified). a a Palpi, b c Glossarium (representing the tongue). d Cultelli or knives (representing mandibles). e. e Scalpelli or lancets (representing maxillae). J Labium.

Order V.-HEMIPTERA (Half-winged), as bugs, aphides, cicadas, lautern-flies, &c.

Characters: Insects having four wings in general, sometimes wanting; mouth a suctorial, beak-shaped organ, adapted to suck the juices of plants and animals, on j, they feed. The Hemiptera proper (or Heteroptera) form a section of this order, and have the basal portion of the anterior wings chitinous or leathery, the remainder membranous. The other important section of Hemiptera is formed by the Homoptera, which have two pairs of membranous wings (or none), the beak springing from the back part of the head. Figs. 32 to 39.

Fig. 32.-Cape Cicada (Platypleura Capensis), Cape of Good Hope (reduced). de Tegmina or parchment-like upper wings.

Fig. 33.-Head of a Cicada. a a The compound eyes. b Upper part of the sucker (representing labrum). f Lower part of the sucker (representing labium). al The vertex. c The epistomium. e The rhinarium. g. Threads contained in the interior of the mouth, representing the maxillae and mandibles.

Fig. 34.—Under-side of a Cicada. a The epistomium. b b Opercula of the musical apparatus or drum-cover.

Fig. 35.-Tesseratoma Chinensis, native of China (natural size). a a Antennae. b Thorax. c Scutellum. d Hemelytra. do Coriaceous or leathery portion of wing. Membranous

Fig. 36. — Lantern Fly (Fulgora Lathburii), East Indies feitei). Fig. ;.—Water Boatman (Notonecta furcata), Europe (natural S126 J. Fig. 38.-Cochineal Insect (Coccus cacti), America (enlarged). a Wingless female. bThe same natural size. c Winged e.

Fig. 39.-Aphis of the rose (Aphis rosae), Europe (enlarged).

Order VI.-QRTHOPTERA (Straight-winged), as the cockroach, locust, cricket, &c. Characters: Insects having four wings, of which the two superior ones are coriaceous or leathery, generally smaller than the posterior, which when not in use fold up like a fan, *ly. Jaws formed for mastication. Figs. 40 O ++.

Fig. 40.-Great Green Grasshopper (Locusta viridissima), native of Britain (reduced). a Upper wings or tegmina, coriaceous. b Lower membranous wings. c Saltatorial legs. d Ovipositor.

Fig. 41.-Head of Do. a Vertex. b Ocelli or simple eyes. c Oculi or compound eyes. d Labial palpi. e Maxillae. f Epistomium. g. Maxillary palpi. h Labrum. iTorulus (base of antennae).

Fig. 42. –Saltatorial Leg of the Locust (Locusta migratoria). a Coxa or hip. b Trochanter. c Femur. d Tibia. eTarsus. f Ungues—claws.

Fig. 43.—Fossorial or Burrowing Leg. Tibia and tarsus of Mole-cricket.

Fig. 44.—Walking-leaf Insect (Phyllium siccifolium), Java and New Guinea (reduced).

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Fig. 1.-Section across the Somersetshire coal-field, showing the occurrence of beds or seams of coal amongst a series of other strata, and which as a whole are called coalmeasures. The appearance of the beds at the surface is termed the outcrop, and the angle formed with the horizon the dip of the strata. The fractures, indicated by the fine lines which interrupt the continuity of the beds, were probably produced by some upheaving force which at the same time gave to the strata, originally horizontal, their present basin shape. These rents, or dislocations as they are sometimes called, are of great importance to the miner, and may be beneficial to him or otherwise according to circumstances; they are frequently called dykes, and are known as upthrow or down-throw dykes according as the edge of strata appears to the observer to be higher or lower in regard to his own position; they are also called shifts or slips, but the common terms are faults or troubles. A line of fault thus cutting off the coal-seam often forms the underground boundary of a colliery, as shown in figures 3 and 4. Fig. 2.-Vertical section of a coal-pit, showing more in detail the various alternations of strata passed through in sinking. Even in the most productive areas the thickness of the coal-seams forms but a small part of the whole of the beds or measures with which they are associated, namely, indurated clays or shales, blue bands of the miner, and sandstones, usually distinguished as rock, metal, organister, and nodular or balls ironstone. The bed immediately below a coal-seam is often penetrated by the roots of plants and trees, showing it to be the soil in which the vegetation whose debris formed the coal grew. These are known as underclays, or when siliceous as ganister. Beds of limestone are occasionally found in the coal-measures, but they are rare. Fig. 3.-In working coal-seams there are two general methods or systems adopted, namely, the pillar-and-stall, or stoop-and-room system, and the long-wall or long-work system. The former is shown in the plan, and consists in driving a series of passages named by the miner stalls or rooms (the principal driving or roadway, being called a bord or gate) in such a manner as to divide the coal into rectangular pillars or stoops, resembling the blocks of buildings and streets in a town. The coal is extracted from each pillar in succession, beginning with those most distant from the shaft. On the removal of the pillars the area is abandoned and the roof falls in, its ruins, together with the waste coal and rubbish, forming what is called the goaf or gob. Fig. 4.—Plan to illustrate the long-wall method of working, where the coal is got at one operation, either by working from the shaft towards the rise of the coal and making safe roadways through the fallen roof by strong stone pillars or walls, or by first driving galleries to the extreme boundary and working the coal back towards the shaft, leaving the goaf or rubbish behind, thus avoiding the necessity of keeping good the roads. Where the goaf is not dangerous from the presence of gas, and the roof is strong, this method is both economical and efficient, as the whole of the coal is at once removed on a long face, and is not subject to the partial crushing that takes place when pillars are left. The long-wall system is more frequently adopted in the working of thin seams. Fig. 5 is a diagram section through the engine-shaft or pit of a colliery, giving a general notion of the walling and tubbing of shafts, and showing also the various parts of the pumping arrangements. When a shaft passes through soft or insecure strata it is either walled round with brick

joints bevelled to the centre of the opening; this is called walling, as shown at a b. If water is met with issuing from the strata in considerable quantities a means of damming it back is adopted called tubbing, shown at c d. This consists in lining the shaft with an impermeable casing of wood or iron, generally the latter, which is built up in segments forming rings placed upon each other throughout the depth of the water-bearing beds. The principal parts of the pumping arrangements shown in the figure are as follows: FFFF force-pumps, PP pumprods, w w water boxes at each lift, LL list or bucket pump, B wind-bore of ditto, D D ladders for the use of the pump men. Figs. 6 and 8 illustrate two of the various forms of safety detaching hooks used in some collieries to guard against accidents arising from overwinding, which causes the rising cage to be brought violently into contact with the head-gear, thereby breaking the rope and otherwise causing serious damage. The safety hook is so constructed that in the event of its coming in contact with a plate or framework fixed to the upper part of the pit frame a suspension bolt is withdrawn, thereby detatching the cage from the rope and arresting its progress, its fall being prevented by safety clutches which are attached to the cage for the purpose. Fig. 7.—Section across the long-wall face or front of a coal working. The whole coal A is called the bank. It is worked away by a party of men called hencers. A deep groove about 9 inches wide is cut or holed in the lower part of the seam next the floor or pavement along the working face, the overhanging coal being for the time supported by short timber props, or blocks of stone called cogs. When ready for removal the props are knocked away, and the mass of coal either falls by its own weight or is forced down by means of wedges or by blasting. It is then broken up and conveyed to the pit bottom, the small unsaleable coal called slack and the rubbish being thrown behind the men, and forming the goaf or gob. A strong wall c is built for the protection of the ways or bords in the immediate neighbourhood of the working. In most cases a double row of timber or iron props, sometimes called ouncheons, FF, is placed about 2 or 3 feet apart, in order to protect the men more securely, as well as those parts of the working through which access is gained to the pit. As the work progresses these supports are removed and placed in advance, the roof behind being allowed to crush entirely 1n. Fig. 9 illustrates what is called a creep in coal-mining If the roof be of hard material, such as sandstone, and the pavement or floor consists of a soft fireclay, the weight of superincumbent strata communicated through the pillars to the floor will often cause the latter to crack and swell up into the passages, spreading sometimes, in spite of attempted remedies, from point to point over a whole district, and ultimately destroying the working. From this cause valuable collieries have been totally ruined and abandoned. Fig. 10.-In this figure is shown the effect of sits or thrust, likewise caused by pressure of the strata; it is the reverse of creeps, the floor being hard and the roof weak, the latter crushes in, and, unless it can be prevented by timbering, eventually fills the passages and ruins the colliery. Figs. 11 and 12 show a side and cross section of Guihal's Ventilating Fan. This machine, the invention of M. Guibal of Liége, is one of the most successful apparatus of its kind which has of late years come into use for the purpose of ven. flat boarded blades, forming a fan which has been made as much as 40 feet diameter, and is revolved by means of steampower within a closed casing of brickwork. The air is drawn up the shaft of the mine through the centre of the fan, as shown by the arrows in fig. 12, and is discharged into a chimney of gradually increasing dimensions towards the top. The size of the discharge aperture can be regulated by a movable shutter sliding in grooves formed to the shape of the casing.


The largest of these machines is capable of exhausting as

much as 200,000 cubic feet of air per minute. Fig. 13 illustrates in a diagram plan the principle adopted for regulating the ventilation of a mine. The arrows indicate the course of the air-currents from the downcast to the upcast shaft, through a furnace, which in this case circulates the current. The lines drawn at A and B from one pillar to another show the position of trap-doors and partitions, or stoppings as they are called, placed across the passages to prevent the air-current from diverging to the upcast before it has passed through the more distant workings. C C are temporary wooden partitions, or brattices, placed at points where it is necessary to direct the air to the face of a working where men are engaged. Figs. 14 and 15 represent the usual form of a ventilating furnace, which is considered to be the most efficient and reliable method of ventilating collieries. The furnace itself consists of a plain fire-grate placed under an arch, and communicating with the upcast shaft by means of an inclined drift, called the furnace drift, as shown in fig. 14. The furnace is placed at a distance of from 30 to 40 yards from the bottom of the pit; the object of the inclined drift intervening is to enable the air to get uniformly heated before passing into the shaft, thus promoting a regular upward movement of the whole column in its passage to the surface. In fiery mines the return air from the workings is usually led into the upcast shaft through a higher passage away from the furnace, called the dumb drift, shown in the figure, as it would not be safe to allow it to pass over the furnace fire— fresh air being conducted for the purpose of combustion through other galleries. Fig. 15 is a front view of the furnace, showing the general form of the arches and side passages. Figs. 16 and 18.-Two forms of the miner's safety-lamp. To guard against explosions of fire-damp in collieries Sir Humphry Davy in 1816 invented the safety-lamp. The common Davy, fig. 18, is a very simple contrivance, consisting of a small oil lamp surrounded by a cylindrical casing of fine wire-gauze, through which material flame will not pass. This gauze cylinder, which has a flat top of the same substance, is secured within a cage composed of 3 or 4 brass wires fitted into a brass ring at each end; the oil vessel is screwed into the lower ring, and the act of screwing locks the lamp, and fixes the gauze so that it cannot be removed without a key. The lamp is carried by a handle attached to a brass plate at the top. There are many modifications of the Davy-lamp; among them the Stephenson, or Geordic, lamp, fig. 16, is one of the safest. It is rather larger and

heavier than the Davy, and is provided with a glass cylinder or chimney within the gauze casing next the flame, the air

for combustion is admitted through a series of small holes

just below the level of the wick, as shown by the arrows in the figure. The light of the Stephenson lamp will go out if the air of the mine becomes dangerously explosive.

Fig. 17 represents in a general way the arrangements adopted for drawing coal from two pits. The engine is placed midway between the shafts, and drives a large drum called the winding drum, on which the ropes are wound in opposite directions, and carried over pulleys down each shaft, thus with their respective cages exerting a counterbalancing effect upon each other. The pit-head, or head-stock, is usually of timber, sometimes of wrought-iron, constructed of two uprights and two back legs strongly framed together, for supporting the guide pulleys or sheaves, G. G. The landing-stages, B B are generally raised some 20 feet above the ground, and are floored with cast-iron plates. When the cage reaches the surface the tubs of coal are pulled on to the platform, and the coal discharged into the screens or sieves, to separate the small from the large coal, the empty tub being pushed into the cage again from the opposite side. During the operation of landing the tubs the cage is kept in position at the stage by means of the keeps, which are projecting levers attached to the framework of the pit top and on which the cage rests.

Fig. 19.-A side view of Whitelaw's Safety Catch. To prevent accidents from the breaking of the rope in the process of winding various forms of safety catches have been adopted; these consist of variously constructed levers, or cams, attached to the cage, which are made to grip the guiderods in the event of the cage being separated from the rope. The figure illustrates a simple and efficient form of such apparatus. The levers A A are kept open by the tension of the winding rope, so as to clear the guides, and when released by the parting of the rope the lower ends are brought up firmly against the sides of the guide-rods, the action being assisted by powerful springs fixed above, and shown at B B; the grip thus maintained is sufficient to hold the cage with its load securely in the shaft until assistance is obtained and a fresh rope attached.

Fig. 20.—A side elevation of the winding cage, which consists of one or more platforms connected together by a light framework of wrought-iron or steel bars, and fitted with iron slides made to fit loosely on three sides of the guide-rods. The cages traverse up and down the pit and receive the tubs or trams of coal. They are constructed with one or more platforms according to the number of tubs they are required to carry, which varies from one to six, or even eight in large collieries. The tubs are wheeled on to a railway formed of angle-iron on the floor of the cage, to which they are securely locked by means of a latch or a bar falling over the ends. When the cage is used for lowering the miners it is provided with a sheet-iron roof as a protection from falling material.

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THE Mollusca, or soft-bodied animals (L. mollis, soft), form or rings. In some the body is naked and unprotected, as the

one of the grand divisions (a sub-kingdom) of the animal kingdom. They are widely distributed, and are commonly known by such names as slugs, snails, limpets, oysters, cockles, whelks, &c. They perform an important part in the grand economy of nature, and furnish food for numerous animals as well as man. They are characterized by having

common garden-slug; in others, however, it is inclosed in a muscular sac, as the tunicaries; but the great majority have the body protected with a calcareous shell, which takes a variety of forms, always interesting, and frequently highly beautiful. It is to those shelled or testaceous mollusca that we shall chiefly confine ourselves in the present paper, which mutual relation of animal and shell, and to explain the prin


cipal terms used by modern writers on the mollusca. Molluscs

that have no shell are called naked. The Shell-bearing molluscs, or Shell-fish, are popularly

divided into UNIVALves, Bivalves, and MULTIVAlves. The

Univalves are those whose shell consists of only a single piece, often open and cup-shaped, as in the limpet, or more commonly of a long cone wound spirally round a real or imaginary axis, as the garden-snail, the whelk or periwinkle. The Bivalves are those of which the shell is formed of two pieces joined by a hinge, as the cockle and oyster. The Multivalves have the shell composed of several pieces. These molluscs are few in number. The shells of the mollusca are secreted by the soft integument or mantle (also called the pallium). The chief mass of the shell is made up of carbonate of lime, with a small proportion of animal matter. Externally the shell is covered with a layer of horny matter called the epidermis or periostracum. The shell increases in size by layer after layer being added by the action of special glands at the edge of the mantle. The matter added to the inner surface of the shell is different from that added to the outer, being nacreous in character, and forming what is known as mother-of-pearl. Univalve shells are the well-known and characteristic residence of the molluscous animals classed as Gasteropoda (belly-footed), that move by means of a flat disk on the under surface. The shell of a gasteropod may be cup-shaped or tubular; but most commonly, as already mentioned, it forms a tube wound spirally round an axis. If this axis is solid it is called the pillar or columella; if open, the umbilicus, and the shell is then said to be perforate or umbilicated. The turns or volutions which the cone makes about the axis are termed whorls. These increase in size towards the mouth of the shell, and the last one is frequently as large or larger than the whole of the others together, and is called the body whorl. The part above the body whorl is called the spire. The line of juncture between the whorls is called the suture. In viewing a spiral univalve shell, it is supposed to be placed with the spire uppermost and the aperture in front (see Fig. 4). The point of the spire is then the aper, and the lower part of the aperture the base. The left side of the aperture is the pillar lip or inner lip, and the right side the outer lip. At the lower part of the aperture is frequently a canal or groove, sometimes long, sometimes short, called the anterior canal; and at the upper part another, called the posterior canal: these are for the passage of the siphons for conveying water to the gills or branchiae. When the animal is in motion the shell is carried on its back, with the apex pointing backwards. The head and foot are protruded from the front of the aperture (which is now underneath), the respiratory or inhalent siphon issuing from the anterior canal, and the eaccretory or eachalent siphon from the posterior canal. The foot is expanded, and forms a large flat disk, and the mantle is spread over the lower part of the shell. On the upper part of the foot is placed the operculum, a shelly or horny plate, with which the animal closes the aperture of its shell when it retires within it. On each side of the head are the tentacles, on which are sometimes situated the eyes; but sometimes, as in the snails and slugs, they are fixed on separate pedicels, which have a telescopic motion. Between these is placed the proboscis, which is capable of being protruded or withdrawn, and which contains in some families a retractile membrane or tongue, called the lingual ribbon. This is covered with minute teeth, and is used for rasping hard surfaces and boring holes through shells, in order to get at the animals within. The structure of the tongue in the whelk is very remarkable. (See Fig. 35.) A bivalve shell consists as a rule of two pieces or valves, which inclose the animal, and are capable of being opened or shut. They are held together by a ligament that forms a hinge, which in many cases is further strengthened by shelly teeth, alternating and locking from one valve into the other. The tendency of the ligament is to open the valves, and they are kept closed by strong muscles, called, for that reason,

inside of the shell; and the circumstance of there being one or two, divides the bivalves into two sections, the Momomyarians, or one-muscled, and the Dimyarians, or two-muscled. These muscles are relaxed at the will of the animal, and at its death they of course lose all power, and the ligament then

acts and forces the shell open, as we generally see in dead

shells. The beak or umbo is situated on the upper or dorsal margin over the hinge, and has its point inclining forward, and the ligament posterior—never anterior to it; and by this may be known the position of the animal. The part of the shell anterior to the beak is in general much shorter than the posterior part; but in a few instances it is the contrary. The umbo is the point from which spring all circles, and from which all lines radiate. On viewing or describing a shell, it is supposed to be placed in the same position as Fig. 23, with the hinge side uppermost, and the beak pointing forward. The ligament will then be posterior, and the terms “right valve” and “left valve,” “anterior” and “posterior,” will be applied to the parts in the positions as now placed. The length of the shell will be measured in the direction of the animal, that is, from anterior to posterior; the breadth from the upper or dorsal margin to the lower or ventral margin, and the thickness through the two valves when closed. On the inside of the shell there often appears a more or less distinctly marked line or impression, which shows the line of junction of the muscular edge of the mantle and the shell, and is called the pallial impression (or line). When the animal possesses retractile siphons this line shows an indentation or bay called the pallial sinus. One family of gasteropoda—the chitons—has a multivalve shell, formed of eight pieces, firmly fixed in a muscular border or collar, and capable of motion. The animal has the habit of fixing itself on stones like the limpet. The characteristic organ of locomotion in the typical mollusca is the “foot” already mentioned, which may be modified so as to perform various offices. Its use in the case of the snail is well known. In the cockle it is developed to a great size, and by its aid the animal can perform considerable leaps. In some cases (as the razor-shells) it enables its possessor to burrow rapidly in the sand; while in the mussels, &c., the organ is devoted to the secretion of the well-known beard or byssus, a collection of strong fibrous threads by means of which these animals moor or fix themselves to rocks and stones. In some bivalves (as the oyster), in which the locomotive powers are in abeyance, the foot is rudimentary. In the cuttle-fishes it is represented by the arms or tentacles round the mouth. A distinct stomach, intestine, and anus are generally to be observed in the mollusca. In many there is a masticatory apparatus consisting of an elongated lingual ribbon or odontophore, as mentioned above; and in the majority there is a well-developed liver. The blood is generally destitute of colour, and in the higher molluscs at least is circulated through the body in a system of vessels connected with a central circulatory organ or heart. The typical breathing organs consist of gills or branchiae. In terrestrial gasteropods (as snails and slugs) we find a pulmonary-sac or lung-chamber, to which atmospheric air is admitted for the due aeration of the blood. Eyes as well as organs of touch and hearing exist in the higher mollusca. The eyes are of the most perfect structure in cuttle-fishes and gasteropods; in many forms visual organs are entirely wanting. The Sub-Kingdom Mollusca is divided into two primary sections, the Mollusca Proper or Higher Mollusca, and the Molluscoida or Lower Mollusca. The former have a more highly developed nervous system, a distinct heart of at least two chambers, and generally a muscular foot. The latter have no specialized heart, or only a rudimentary one, and have no foot. The Mollusca Proper are divided into the following four classes: Cephalopoda, Gasteropoda, Pteropoda, and Lamellibranchiata. They may be also divided into the

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Fig. 8-Spider-shell (Pteroceras chiragra). Shell digitate. a Spire, b Body-whorl. c. Outer lip., d Inner lip and columella, e Anterior canal. f Posterior canal in one of

Fig. 9.—Cowrie (Cypraea caurica). Shell Convolute. a Inner lip. c Anterior canal. b Outer lip. d Posterior canal.

Fig. 10. Pleurotoma Babylonica. Eastern Seas. a Notch in outer lip... b Spire, turreted. c Beak, long and straight. d Columella.

Fig. 11.—Keyhole Limpet (Fissurella annulata). a Aperture in the apex for the anal or excurrent siphon.

Fig. 12.—Ear-shell (Haliotis virginea). Shell ear-shaped. a Spire. b. A series of holes for the passage of the two pointed lobes of the mantle.

Fig. 13.-Worm-shell (Vermetus lumbricalis). Shell, upper part spiral; lower whorls detached and irregular. a Spire. b Detached whorls.

Fig. 14.—Tooth-shell (Dentalium entalis). a Anterior. b Posterior.

Fig. 15.—Chiton (Chiton tulipa). Shell multivalve, of eight distinct plates. a Anterior plate. p Posterior plate. m. Border or mantle.

Shell tubular.

Order II.-Opisthobranchiata. Shell rudimentary or wanting; branchiae more or less completely exposed, and placed on the sides or at the posterior of the body, and behind the heart. Examples, Sea-hare, Sea-lemon, &c.

Order III.-Heteropoda. Free-swimming animals; shell present or absent; swim by means of a fin-like appendage. Example, Carinaria.

Sub-Class B.-PULMONIFERA or PULMOGASTEROPODA. Respiration aerial, air being admitted into a pulmonary chamber by an external aperture. Land-snails, Slugs, Pond-snails, &c.

Fig. 16.-Pond-snail (Planorbis corneus). a Head. b Tentacles. c Eyes. d Foot.

Fig. 17. –Helix virgata. A perforate or umbilicated shell. a Peristome, or mouth border, “lunar” in form. b Umbilicus. Fig. 18.-Helix harmastoma. Shell imperforate or not umbilicated. a Outer lip. b Inner lip.

Shell discoidal.

Fig. 19.-Auricula scarabaeus. a Aperture denticulate or toothed.

Fig. 20.—Bulimus decollatus. Spire decollated in the adult.

CLAss III.-PTERoPodA (Gr. pteron, wing, and pous, podos, foot).

Characters: Small free-swimming animals found in immense quantities on the surface of the Arctic and other seas; named from possessing two wing-like fins springing from the sides of the head, and formed by developments of the *. and lateral portions of the foot; shell, when developed, of a symmetrical shape, and of a delicate glassy consistence; breathe by gills or ciliated surfaces, or do not show any specialized respiratory apparatus; all hermaphrodite. A well-known example is the Clio, which forms the chief food of the whales in the Arctic Seas.

Fig. 21. – Limacina antarctica, a pteropod of the South Polar Seas. a Shell sinistrally spiral, that is having the aperture on the left side of the pillar or columella instead of on the right. b Epipodium or wing-like expansion of the foot.

CLAss IV.-LAMELLIBRANCHIATA or CoNCHIFERA. Bivalves, as Mussels, Cockles, Oysters, &c.

Characters: Animals acephalous, that is, possessing no distinct head; body more or less completely protected by a bivalve shell; generally four lamellar or plate-like gills, two on each side of the body, sometimes only one on each side; mouth unprovided with any dental apparatus.

Section A.—SiphoniDA. Respiratory siphons present; lobes

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