REPTILES. 327 



except the theromorphs, by structural characters, although in 

 external appearance they are closely similar to the lizards. 

 They have the vertebrae pro- or amphiccelous or amphiplatyan, 

 the atlas peculiar in consisting of four parts ; cervical ribs are 

 present, and the sternum is largely cartilaginous. The skull has 

 usually a corroded appearance, an interorbital septum occurs, 

 the premaxillse are paired, and there is no parietal foramen. 

 Ventral ribs are present ; the procoracoid is a process of the 

 coracoid. The acetabulum is closed. All of the bones are 

 solid. 



Dermal plates are best developed on the back, but may occur 

 on the ventral surface as well. They consist of dermal ossifica- 

 tions overlaid with epidermal scales. The eyes have a vertical 

 pupil, and both lids and nictitating membrane are present. 

 The nostrils and ears are provided with valves. The teeth 

 are confined to the edges of the jaws, and are never found 

 on palatines or pterygoids. Salivary and lachrymal glands are 

 lacking ; the stomach is muscular and resembles somewhat 

 that of birds. There is no caecum. A peculiarity is the ex- 

 istence of peritoneal canals connecting the coelora with the 

 exterior. 



The crocodiles and alligators are among the largest of living 

 reptiles, the giant tortoises alone rivalling them. They are 

 aquatic and mostly fluviatile. They capture their prey by lying 

 in wait for it, usually with but the eyes and the tip of the nose 

 above water. In their motions they are very quick. The 

 smaller forms live chiefly upon fishes, but the larger prey on 

 mammals when the chance comes. The eggs are laid either in 

 sand or in rough nests, and vary in size from those of a hen 

 to those of a goose. The group to-day is exclusively tropical, 

 and has recently been greatly reduced in numbers, owing to 

 the desire for the skins. The crocodiles appear in the trias 

 (Parasuchia and Pseudosuchia). 



Sub-Order i. Pseudosuchia. 



Extinct crocodilia in which the back is covered with two rows of oblong 

 bony plates. Vertebrae unknown ; premaxilla small and thin ; nostrils an- 

 terior ; postorbitals present ; no infratemporal fossa ; teetli few ; hinder 



