THE EVOLUTION OF THE REPTILES 441 



Triassic Mixosaurus, through the Jurassic Ichthyosaurus to 

 the Cretaceous Ophthalmosaurus. They reached lengths of 

 30 feet. 



All the Squamata which possess limbs have a hook-shaped 

 5th metatarsal. The first group of these are the Lacertilia, 

 first appearing in the Jurassic, and represented now by the 

 lizards, geckos, and chamaeleons. In the Cretaceous, a group 

 of Lacertilia became adapted to an aquatic life, — the Mosa- 

 sauria. They reached a length of as much as 40 feet, and their 

 limbs became modified into paddles. The second group of 

 the Squamata are the Ophidia or snakes. It is characteristic 

 of the Squamata that the quadrate is loose, and in the Ophidia 

 the two halves of the lower jaw are separate, which enables 

 relatively enormous mouthfuls to be swallowed. 



The remaining reptiles form the group Diapsida, for their 

 skull roof is perforated by two temporal fossae or vacuities on 

 each side. So far as is known, all of them have a hook-shaped 

 5th metatarsal. Here belong the Rhynchocephalia, the 

 Crocodilia, the Dinosaurs and the Pterosaurs, and the Diapsida 

 also contained the ancestors of the birds. 



The Rhynchocephalia appear in the Triassic with Rhyncho- 

 saurus, and are represented to-day by Sphenodon. They are 

 primitive in retaining the gastralia. The Triassic Thalatto- 

 saurs, which had paddle-like limbs, were probably related to 

 the Rhynchocephalia. 



The Crocodilia form a large group of generalised reptiles, 

 possibly dating back to the Permian. The Triassic Pseudo- 

 suchia, of which Euparkeria is an example, are regarded as 

 related to the ancestors of the Rhynchocephalia, the Dinosaurs 

 and Pterosaurs, the existing crocodiles, and the birds. The 

 living crocodiles and alligators retain the gastralia, and other 

 dermal ossifications. Mention may be made of the Jurassic 

 Thalattosuchia, yet another group of reptiles which became 

 secondarily adapted to aquatic life with paddle-like limbs. 



The Dinosaurs were the dominant animals in the Jurassic 

 and Cretaceous. The skull had two temporal vacuities on 

 each side, and in addition a prelachrymal vacuity. Some 

 were quadrupedal and herbivorous, such as Diplodocus 



