176 REPTILIA. 



Sub-Order 3. Trionychia. 



The three-clawed mud-turtles appear suddenly with all 

 their typical characters in the lowest Tertiary strata both of 

 Europe and North America. Numerous fine specimens of 

 Trionyx itself are known from the Upper Eocene of Hordvvell, 

 Hampshire, and the genus seems to have survived in Europe 

 throughout the Miocene period. 



ORDER 4. ICHTHYOPTERYGIA. 



As already remarked (p. 142) there is still a fourth well- 

 defined group of primitive reptiles, in which the investing 

 bones of the temporal region of the skull contract into a single 

 broad arcade. These are the Ichthyopterygia (fish-limbed 

 reptiles), which range throughout the Mesozoic period with 

 very little structural modification, and arise in the Trias as 

 abruptly as they disappear in the Chalk. In outward form 

 they must have closely resembled the Cetacean mammals of 

 the present day, having a large head, long rostrum, and no real 

 neck ; but their hind limbs never disappeared (although these 

 were sometimes extremely reduced), and the caudal fin was 

 expanded in a vertical, not in a horizontal plane. Like the 

 Cetaceans, moreover, there is some reason to believe that the 

 Ichthyopterygia were originally derived from land-animals ; for 

 the radius and ulna in the known Triassic types are invariably 

 more elongated and less pressed together than in the later forms, 

 the limb being thus less completely paddle-shaped than in the 

 latter. It is also interesting to note that in the jaws from the 

 Trias the teeth are in a less uniform series than in those from 

 the Jurassic and Cretaceous ; while a few of the Upper Jurassic 

 and Cretaceous forms are almost or completely toothless. Some 

 of the later paddles, too, are broader and rendered more flexible 

 than those of earlier date by the persistence of a considerable 

 layer of cartilage round their carpals, tarsals, and phalanges. 



What the land-dwelling ancestors of the Ichthyopterygia 

 may have been, it is still impossible to determine. It is, 

 however, worthy of remark that these reptiles exhibit a vertical 



