Page:EB1911 - Volume 23.djvu/161

Rh found (Proganochelys). Its members are proved to have been, toothless since the Jurassic period, and have only changed very slightly since their first appearance. The marine turtles seem to have first acquired elongated paddles and vacuities in the shell during the Cretaceous period, and the Trionychia, destitute of epidermal shields, apparently arose at the same time.

Order 3. —These are amphibious or aquatic reptiles (fig. 4). The head is comparatively small in most genera, and the neck is usually elongated though not flexible. The tail is insignincant, generally short, and both pairs of paddles seem to have been concerned in progression. The order appears to have arisen from a group of land-reptiles, for its earliest members, from the Triassic of Europe (Lariosaurus) and from the Permo-Carboniferous of S. Africa (Mesosaurus) and Brazil (Stereosternum), are all amphibious animals. They are comparatively small, and their limbs are only just becoming paddle-like. The skull suggests affinities with the terrestrial Anomodontia, and the shape of the scapula seems to show some connexion with the Chelonia. The truly aquatic Sauropterygians of the Jurassic (fig. 4) and Cretaceous periods possess most

effective paddles with elongated digits, and as the genera are traced upwards in the geological formations it is possible to observe how the arches supporting the limbs become more rigid until the maximum of strength is reached. A few genera, such as Pliosaurus from the Jurassic and Polyptychodon from the Cretaceous of Europe, are distinguished by their relatively large head and stout neck. Some of the largest Upper Jurassic and Cretaceous species must have been 10 metres in length. They were cosmopolitan in their distribution, but became extinct before the dawn of the Tertiary period.

Order 4. —The Ichthyosaurians are all fish-shaped, with a relatively large head and very short neck. Both pairs of paddles are retained, but the hinder pair is usually very small, and locomotion seems to have been chiefly effected by a large caudal tin. This fin, as shown in impression by certain fossils from Württemberg and Bavaria, is a vertical, triangular, dermal expansion, without any skeletal support except the hinder most part of the attenuated vertebral column, which extends along the border of its lower lobe (fig. 5). Another triangular fin, without skeletal support, is known to occur on the back, at least in one species (fig. 5). Some of the genera are proved to have been viviparous. Like the Sauropterygia, the Ichthyopterygia appear to have originated from terrestrial ancestors, for their earliest Triassic representatives (Mixosaurus) have the teeth less uniform and the limbs slightly less paddle-shaped than the latter genera. In this connexion it is noteworthy that their hollow conical teeth exhibit curious infoldings of the wall, like those observed in many Labyrinthodonts, while their short, biconcave vertebrae almost exactly resemble those of the Labyrinthodont Mastodonsaurus and its allies. As the Ichthyosaurs are traced upwards in geological time, some genera become almost, or quite, toothless, while the paddles grow wider, and are rendered more flexible by the persistence of cartilage round their constituent bones (Ophthalmosaurus). They were cosmopolitan in distribution, but disappeared from all seas at the close of the Cretaceous period. The largest forms, with a skull 2 metres in length, occur in the Lower Lias.

Order 5. —These are small lizard-shaped reptiles, which have scarcely changed since the Triassic period. Though now represented only by Sphenodon or Hatteria, which survives in certain islands off New Zealand, in the Mesozoic epoch they ranged at least over Europe, Asia and North America. They comprise the earliest known reptile, Palaeohatteria, from the Lower Permian of Saxony, which differs from the Triassic and later genera in having an imperfectly ossified pubis and ischium, more numerous abdominal ribs, and the fifth metatarsal bone normal. They are also represented in the Permian, chiefly of North America, by the so-called Pelycosauria, which have sharp teeth in sockets, and are remarkable for the extreme