TRANSITIONAL FORMS.

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the higher vertebrates; they are partly aquatic and partly, as their name indicates, amphibious. Several aquatic amphibians are closely related in structure to the Lepidosiren (or mudfish) of the Australian and African rivers, a type which has been lately shown by Dr. Günther to be nearly akin to Ganoids through the newly discovered genus Ceratodus- a fact foreseen by Professor Huxley, who had already pointed out its affinities with the most ancient Ganoids of the Paleozoic era. Again, some extinct labyrinthodonts, frog-like amphibians of large size with ventral armour-plates like alligators, have skulls approaching ganoid fish, whilst others are peculiarly reptilian in their characters.

Reptiles are the most diverse in form, and, in the majority of cases, the least-highly differentiated, of terrestrial vertebrates; they are probably the descendants of types strictly transitional between the mammal, bird, and amphibian; and they exhibit a vast number of marine aquatic and semiaquatic forms; the extinct Ichthyosauria and Plesiosauria, the existing tortoises, turtles, and Crocodilia, and the semiaquatic lizards and snakes are sufficiently well-known examples.

Amongst birds the penguins and auks are highly aquatic; and an endeavour will be made to show that they exhibit remarkable reptilian tendencies.

There are two great types of birds at present existing on our globe :-the Ratitæ, or Ostriches and Cassowaries (great land birds with rudimentary wings and enormously developed legs); and the Carinatæ, or keel-breasted birds, which include all the other bird forms. The latter are decidedly the most differentiated in a special direction, and as a subclass have the fewest reptilian characters. The penguins belong to the carinate subclass, and divide the claim to a transitional position between birds and reptiles with the Ratitæ, or ostrich form. There can be no doubt that the sternum and shoulder-girdle of the Ratitæ are remarkably like those of some Reptilia, whilst the rudimentary cervical ribs and prepubic bones of the ostrich are unmistakably crocodilian. It is, however, important to remember that the skull of the carinate tinamou is certainly more reptile-like than that of any other bird, although other parts of its organism are highly differentiated on the bird type.

The scarcely united tarso-metatarsal bones of the penguins and their near allies the frigate birds are strongly reptilian characters, as are also the large fibula and broad expanded scapula and solid bones of these birds. In one of the penguins (Pygosceles) the coracoid has a large fontanelle, as it has in the Ratitæ, a reptilian character, dividing the bone into

ICHTHYORNID BIRDS.

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a coracoid and precoracoid; this condition exists in a less degree in the king penguin. Again, the proximal tarsal bone is less intimately united with the tibia than it is either in the ostriches or in the aërial birds-another eminently reptilian character.

In the penguins the wings (which are perhaps the most characteristic organs of birds) are rudimentary, as they are in the ostrich form, and more especially in the Apteryx. There is this great difference, however, that in the penguins they are of high functional importance and are used in diving, just as the turtle uses its anterior paddles for progression through the water; whilst those of the ostrich family only preserve the rudiments of a function, and in the Apteryx at least are quite useless. They certainly have the appearance of aborted rather than newly acquired organs.

The view that the aquatic penguins belong to an early type of birds has been materially strengthened of late by Professor Marsh's remarkable discovery of an Ichthyornid type of birds in the Cretaceous shales of Kansas with amphicœlous vertebræ, a carinate sternum, and posterior extremities closely resembling those of swimming birds. Not only

have the remarkable fossils on which his description is founded the amphicœlous vertebra of a fish or more properly of a Rhynchosaurian lizard, a type of reptile with characters tending to the bird type,

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AQUATIC MAMMALIA.

but they have jaws furnished with numerous minute conical teeth *.

Passing from aquatic birds to the aquatic mammalia we find far more diverse forms; but, complex as the subject is, the inference that the earlier types were aquatic is well borne out.

Two remarkably reptilian types of mammals exist, the duck-billed Platypus and the Australian Anteater; one of these, and the more reptilian of the two, is entirely aquatic.

Amongst the higher Mammalia, the Cetacea or whales and the Sirenia or herbivorous whales are highly differentiated forms; and these at first sight appear to form an exception to the view that the aquatic are less highly developed and less differen

* The following are the principal characters of this remarkable fossil type, according to Professor Marsh :

Skull of moderate size; eyes placed well forward. Mandible long and slender; rami not united at the symphysis, abruptly truncated behind the articulation of the quadrate, not encased in a horny sheath. About twenty conical teeth implanted in sockets in each side of the mandible and maxillæ; scapular arch, wings, and legs those of a bird. Vertebræ amphicœlous. Sternum carinate. Bones of the posterior extremity resembling those of swimming birds. Last vertebra of the sacrum unusually large; tail unknown. Bones, except those of the skull, not pneumatic. Ichthyornis dispar of Marsh was about the size of a pigeon. Another form, Aptornis celer, has also been discovered. The skeletons are described in the American Journal of Science and Arts,' vol. v. 1873.

tiated than the terrestrial forms. That these vast animals have passed far from their primitive type form is indubitable; but they still exhibit resemblances to reptiles that are as puzzling as they are unmistakable-not to the reptile of to-day, but to the great extinct ichthyosaurian. Passing over the general resemblance of form, which is not in itself important, the non-union of the majority of the ribs and sternum, the peculiar articulation of the ribs with the vertebræ, the remarkable sternum itself, the chevron bones of the caudal region, the late union of the neural arches and bodies of the vertebræ, the long symphysis of the lower jaw, the remarkable teeth, and the absence or extremely rudimentary condition of the pelvis are either reptilian or ichthyosaurian tendencies.

Palæontology undoubtedly reveals several transitions between the ordinary Cetacea and Sirenia, as well as between these orders and terrestrial forms belonging to exceedingly highly differentiated and diverse orders. Nevertheless the foregoing considerations seem to indicate that the whales form no exception to the views advocated here, and that they are rather to be looked upon as the descendants of marine ancestors of existing terrestrial forms than as the descendants of a highly modified terrestrial type.

The seals may probably be safely viewed in the