CROCODILES, ALLIGATORS, G A VIALS. 593 



while the head was very large. In both, the limbs form powerful 

 elongated paddles, with apparently no trace of nails. 



Ichthyopterygia. Large marine carnivorous Reptiles, represented 

 from the Trias to the Chalk, with tapering body like that of a shark, 

 large dorsal and caudal fins, and two pairs of paddle-like limbs. There 

 is no exoskeleton. The length of the body is sometimes 30 to 40 feet. 

 In the paddle the number of digits is often more than five, and the 

 phalanges of each are often very numerous. The skull has a large 

 parietal foramen, and shows other affinities with that of Sphenodon. 

 Some species were apparently viviparous. 



Examples : Ichthyosaurus, Ophthalmosaurus. 



Mosasauria. These strange Cretaceous Reptiles should probably be 

 placed between the Lacertilia and the Rhynchocephalia. They are 

 specially characterised by the enormous elongation of the body, which 

 sometimes reached a length of 75 to 80 feet. The skull is like that of 

 the Monitor among the lizards, but according to Cope it also presents 

 affinities with snakes. The body is snake-like, but there are two well- 



FIG. 205. Comparison of pelvic girdles of Cassowary 

 (to left) ; Iguanodon, an extinct Reptile (in centre) ; 

 Crocodile (to right). 



//., Ilium ; Is., ischium ; P., pubis. 



developed pairs of limbs, forming swimming paddles. All were car- 

 nivorous and marine ; the distribution was cosmopolitan. 



Mosasatirus, Clidastes, Liodon. 



Dinosauria. Terrestrial Reptiles, ranging from the Trias to the 

 Chalk, often very large, and, like Marsupials, specialised in various 

 directions. They exhibit many points of resemblance to Crocodiles on 

 the one side and to Birds on the other. Brontosaurus, a gigantic, 

 herbivorous form, nearly sixty feet in length was probably amphibious. 

 Atlantosaurus was even larger, the femur measuring over six feet in 

 length. Compsognathus, Iguanodon, and Camptosaurus are examples 

 of the " bird-footed " herbivorous Dinosaurs. In all these the form 

 of the pelvis and of the hind limbs presents very strong affinities 

 with the conditions seen in Birds. Compsognathtts only reached a 

 length of two feet, and hopped on its hind legs like a bird. Iguanodon 

 habitually walked on its hind limbs, and, like several others, had hollow 

 bones ; it reached a height of fifteen feet. Of the carnivorous Dinosaurs, 

 38 



