Fig. 22.-Venus Shell (Cytherea dione), right valve.
Fig. 23.-Venus Shell (Cytherea morphina), profile.

a Hinge. b Cardinal teeth or central teeth of the hinge. c Lateral teeth. d Upper or dorsal margin. e Lower or ventral margin. f Umbo or beak. 9 Ligament. hh Adductor muscles. Pallial impression. Pallial sinus. 7 Escutcheon, a depressed space behind the umbo. m Anterior end. n Posterior end. o Lunule, a small depressed space before the umbo. p Right valve. q Left valve. mn Length. de Breadth. pq Thickness.

Fig. 24.-Piddock or Stone-borer (Pholas dactylus), bivalve with accessory valves or plates.

a Right valve. Left valve. c Umbonal valves. d Postumbonal valves. e Dorsal valve.

Fig. 25.-Sunset-Shell (Psammobia tellinella).

a Foot. b Excurrent siphon. c Incurrent or respiratory siphon.

Fig. 26.-Trough Shell (Mactra Brasiliana); Brazil.

a Cartilage or ligament lodged in a pit among the teeth. Fig. 27.-Gaper Shell (Mya truncata).

a Left valve, smaller than right. b Valves truncated. Fig. 28.-Lantern-Shell (Anatina hispidula); Egypt. Valves gaping; umbones directed backwards; hinge with a free ossicle.

Fig. 29.-Ribbed Cockle (Cardium costatum); West Africa. Ribs radiating from umbo.

Section B.-ASIPHONIDA. No respiratory siphons; lobes of mantle not united.

Fig. 3). Scaly Pinna (Pinna squamosa). Shell wedgeshaped.

a Hinge. b Byssus. c Squamæ or scales.

Fig. 31.-Ark-Shell (Arca granosa); Australia.

a Hinge line, straight with numerous transverse teeth.

Fig. 32.-Hammer-Shell (Malleus vulgaris). Valves of a strangely crumpled form, and expanded into long process in the line of the hinge.

Fig. 33.-Wing-Shell, Pearl Oyster (Avicula heteroptera). Valves unequal; hinge-line straight and very long.

The above divisions comprise all the Higher Mollusca. The Lower Mollusca form three classes, the Brachiopoda, Tunicata, and Polyzoa, only the first of which resemble in form the characteristic mollusca or shell-fish with which we are familiar.

CLASS V.-BRACHIOPODA (Gr. brachion, arm, pous, podos, foot). Lamp-shells (so called from resembling ancient Roman lamps); mostly fossil.

Characters: Animals enclosed in a bivalve shell, the valves of which are not right and left (as in Lamellibranchiata), but dorsal or upper and ventral or lower; mouth furnished with two long arms bearing cirri, and coiled spirally in repose; ventral valve often perforated by an orifice through which a muscular stalk or peduncle passes; ventral valve generally the larger of the two.

PALEONTOLOGY is that department of geological science which treats of the animals and plants that existed in remote ages on the surface of the globe. It investigates the petrified or fossil remains of these animals and plants, organisms which belong, with a few exceptions, to species that have long been extinct. Geologists classify the strata containing organic remains, according to the nature and succession of their fossils, under three great epochs, or life periods, viz. Paleozoic, or ancient life period; Mesozoic, or middle life period; Kainozoic, or recent life period. The systems or formations, defined and separated by their organic characters, and representing distinct periods of time, are arranged as follows under these three epochs:

schists, &c., appear to be destitute of organic remains. Some geologists believe that these rocks were deposited before the creation of organized beings; others, with greater reason, are of opinion that the extreme heat to which they have been subjected, and which has rendered them highly crystalline; was sufficient to efface the vestiges of organic structure. The earliest fossils are discovered in the Cambrian and Silurian systems. In the lower beds of these systems sponges, fucoids and annelids are found; the Silurian is rich in mollusca; and the remains of fishes appear in the upper beds. The graptolite, or sea-pen (fig. 4), represents numerous zoophytes which have left their remains in the Silurian strata, not unfrequently embedded in anthracite, containing the vestiges of sea-weeds of the fucoid type, and no doubt deriving its origin from the remains of marine algæ. The lowest bed of the same sytem abounds with the lingula (fig. 1), one of the brachiopoda, or arm-footed class of molluscs; the upper beds contain species of Pentamerus (fig. 3), belonging to the same class, and occurring in all the strata below the carboniferous or mountain limestone. The othoceras (fig. 5) is a chambered shell, which may be compared to a nautilus uncoiled, and represents in these primary strata the highest class of molluscan animals, namely, the cephalopods. But the most characteristic animal of the Silurian is the trilobite, a specimen of which (Asaphus tyrannus) is shown in fig. 2, and Bronteus flabellifer (fig. 8), belonging to a family strictly of the paleozoic age, the last of the species disappearing with the carboniferous limestone. It was an articulated animal of the crustacean

constructed on the same plan as those of modern crustaceans and insects; in the species of Asaphus figured, the eye is computed to have consisted of 6000 distinct facets or visual surfaces, each belonging in a sense to a distinct eye. The rocks of this group contain representatives of all the great existing types of animal structure, up to the vertebrate. The system known as the Devonian, or Old Red Sandstone, is particularly characterized by the remains of fishes of complex and peculiar structure, of which the pterichthys (fig. 6), named in honour of Hugh Miller, its discoverer, may serve as an example. The fishes of that age, amongst other peculiarities, were characterized by being encased in a covering of hard, shining plates called ganoid plates, and hence their name of ganoid fishes. Reptilian remains also occur in the system; and the brachiopods are represented by Calceola sandalina (fig. 7), which is peculiar to the Devonian.-The Carboniferous or Coal System is remarkable for its wealth of vegetation, entitling the age to be pre-eminently regarded as one of plants. There are about 300 species of fossil plants in the British coal-measures alone, of which a considerable proportion are ferns (fig. 11) of various species, probably arborescent; calamites (fig. 12), reed-like plants, usually found compressed; lepidodendra, huge trees, seemingly allied to our comparatively tiny club-mosses; and sigillaria and stigmaria (fig. 10), the former the stem and the latter the creeping-roots or rhizomes of trees to which no living plant bears any close affinity. Nearly the same species prevail throughout all the coal-fields of the world, and the above may be considered as having contributed the greatest part of the woody matter, which, under the influence of heat, pressure, and chemical action, has been converted into coal. The coal-measures contain fresh-water or estuary shells (Anthrocosia, Anthrocoma, &c.) A few reptilian (saurian) remains have been yielded by the system, together with several insects. The subordinate rocks of the carboniferous or mountain limestone, an eminently marine portion of the system, are rich in brachiopod shells, of which the Productus giganteus (fig. 13) and Spirifer striatus (fig. 14) are examples. In the same productive limestones there are numerous representatives of the lamellibranchiate class of molluscs (to which the modern mussel and oyster belong); of the gasteropods, or belly-footed class (like the whelk and limpet); there are numerous species of Euomphalus (fig. 9), of Bellerophon (fig. 15), and various genera of cephalopods, the principal of which are Goniatites (fig. 16), Nautilus, and Orthoceras. Many species of fish, powerful and predaceous, have been described.-The Permian System is not profuse in fossils. Of its fishes, the genus Palæoniscus (fig. 18) combines with rhomboid scales, a heterocercal tail, in which the vertebral column is prolonged into the upper lobe. Schizodus (fig. 17) is one of the lamellibranchiate molluscs of the Permian. Amphibia and reptiles are also among the fossils of this system.

II. MIDDLE LIFE PERIOD (Mesozoic).-The transition is broadly marked betwixt the Paleozoic and Mesozoic life periods. The latter is introduced by the Triassic System, containing fossils more nearly related to the newer rocks above than to those of the older formations below. Footprints of a huge batrachian or frog-like animal, named labyrinthodon (fig. 19), have been discovered in the rocks of the system in Cheshire, in Dumfriesshire, in Saxony, and on the Connecticut river in America. In the Triassic rocks at Würtemberg the remains of a mammalian marsupial animal, named Microlestes antiquus, have been detected, being the earliest discovered mammalian relic hitherto recorded.-The Oölitic and Liassic System indicates an age of reptiles. Upwards of thirty species of ichthyosaurus, or fish-lizard (fig. 23), have been discovered. It has been described as possessing a porpoise's snout, a crocodile's teeth, a lizard's head, a fish's vertebræ, and a whale's paddles. The animal's eye was of great magnitude. The plesiosaurus (fig. 24), a contemporary of the former saurian, had a lizard's head, a crocodile's teeth, a neck of extraordinary length, and a trunk and tail having the proportions of one of the larger quadrupeds, the ribs of a

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(fig. 25), a winged reptile, was suited to fly in the air or swim in the water. In the Lias, the cephalopod class of molluscs is largely represented by ammonites (fig. 22), belemnites, and nautili. The Gryphaa incurva (fig. 20) exhibits the graceful form of one of the oysters of the age; and Aricula cygnipes (fig. 21) is a pearl-oyster. The deep-bodied fish, Dapedius politus (fig. 19a), is one of the numerous fishes of the system. The Stonesfield slate overlying the Lias has revealed the jaw-bones of several species of small insectivorous marsupial animals, of which Phascolotherium Bucklandi (fig. 28), a small insectivorous marsupial, is an example. The Oölite deposits contain numerous bivalve shell-fish, such as pectens, oysters, &c. To the bivalve class also belongs Trigonia costata (fig. 26), of which genus a hundred species are known in the secondary rocks. Species of terebratula, or lamp-shell (figs. 27 and 30), are still conspicuous amongst the declining class of brachiopods. The lower Oölite abounds with gasteropods, including the genus Purpuroidea or Purpurina (fig. 29), an extinct member of the whelk family. The Oxford clay, still higher in the series, is remarkable for the recurrence of many ammonites and belemnites (fig. 31), the latter name being assigned to a family of extinct cuttlefishes of the secondary period. The Oölite contains considerable deposits of coal, which have been profitably wrought in several instances. Figs. 32, 33, 34 are representatives of the fresh-water shells of the upper Oölite. The same rocks have recently yielded mammalian remains; some marsupial, others placental. The Chalk System overlies the Oölite, the freshwater beds of the Wealden occupying an intermediate place betwixt them. These latter deposits contain fresh-water shells, such as belong to the genera Paludina (fig. 35) and Cypris (fig. 36). But the characteristic fossils of the Wealden are saurian reptiles of extraordinary dimensions-the megalosaurus, hyælosaurus, iguanodon, &c. Overlying the Wealden beds is the lower Greensand, amongst the fossils of which are many echinoderms allied to the sea-urchin (for example Salenia punctata, fig. 37); and a distinctive molluscan form is Perna mulleti (fig. 38). The Gault clay, another deposit subordinate to the Chalk, contains the genera Turrilites and Hamites (fig. 39), species of cephalopods allied to the ammonite. The Chalk formation is marked by unambiguous characters; its structure is uniform, and its organisms are eminently marine, consisting of sponges, corals, echinoderms, molluscs, crustaceans, fishes, and saurian reptiles. Amongst the echinoderms are species of Galerites (fig. 40) and Nucleolites (fig. 41), associated with which are numerous star-fishes, including an extinct species belonging to the existing genus Goniaster (G. Coombii, fig. 42). The brachiopodous molluscs are represented by rhynconella (fig. 43) and terebratula; belonging to the ammonite alliance are Turrilites (fig. 44), Baculites, and Scaphites (fig. 45). The system contains about 800 species of animals, with traces of sea-weeds.

III. TERTIARY LIFE PERIOD (Kainozoic).—In this period the forms of animals and plants gradually approximate towards those of existing genera; and the formations of this last great geological epoch have been classified in accordance with the greater or less resemblance of their contained organisms to those of living nature. The Eocene group (Gr. ēōs, dawn, kainos, recent) represents the dawn of the existing flora and fauna. Its plants are exemplified by the fruits of tropical palms, such as Nipadites (fig. 46), found in the island of Sheppey. The London clay has yielded an instructive suite of fossils of the age. Many of the molluscs have a tropical aspect; in the fluvio-marine deposits of Hampshire and the Isle of Wight occur such instances of this character as Voluta spinosa (fig. 47), Fusus porrectus (fig. 48), Oliva Branderi (fig. 49), and Nummulites lævigata (fig. 50). Fresh-water beds of the time contain Planorbis euomphalus (fig. 51). In associated strata are obtained Paludina lenta (fig. 52), Limnæa longiscuta (fig. 53), and several other fresh-water and brackish-water species. The Eocene rocks near Paris produced nearly fifty extinct species of mammalian animals, which were described by Cuvier; they were chiefly pachy

therium, &c. Fossil birds were detected in the same formation. The Miocene (Gr. meion, less), containing a higher proportion of living species than the previous group, abounds in the remains of the elephant, rhinoceros, bears, and numerous other quadrupeds.-The Pliocene (Gr. pleion, more) makes a still greater advance towards the present state of nature. Of its numerous mollusca a considerable proportion now live in British seas; and one of the most characteristic of these is Fusus contrarius (fig. 54). Skates and sharks have left their teeth in the same formation; fig. 55 represents the tooth of a shark (Carcharodus megalodon). The bones of whales also occur, including the ear-bone (fig. 56), together with a profusion of animal exuviæ, now employed in the manufacture of manure.-The Post-Pliocene group belongs to the geological era immediately preceding the existing age. The deposits point to the prevalence of an arctic climate in many parts of Europe and America, and the traces of iceaction are frequent in the British Islands. To floating

glaciers, or to a current of water flowing from north-west to south-east, have been ascribed the enormous deposits of drifted matter, containing boulders or rolled stones. To the descent of glaciers in valleys are assigned the grooved and polished surfaces of rocks observed in mountainous districts, and particularly in the Highlands of Scotland. In the beds of the drift are found the bones of the Elephas primigenius, or mammoth (fig. 57); the remains of this animal being widely dispersed over Europe, Asia, and North America. To the post-glacial age belongs the Irish elk (fig. 58), which has left its remains in Ireland, Scotland, England, and France. Among its contemporaries were the reindeer, oxen, beaver, wolves, &c. It only remains to be stated that, associated with the drift or boulder clay, are marine beds, first observed in the valley of the Clyde, containing several shells which now only exist alive in arctic seas. The shells found in the raised beaches seen on our shores are exclusively of existing species.