xni PHYLUM CHORDATA 313 



sacrum consists of two vertebrae. Numerous intercentra are 

 usually present. The ribs have simple vertebral extremities, 

 and are provided with uncinates. There is a system of abdominal 

 ribs. The quadrate is immovably fixed to the other bones of the 

 skull. There are both upper and lower temporal arches. The 

 rami of the mandible are united by ligament. There is a sternum. 

 The teeth are acrodont. The lungs, heart, and brain resemble 

 those of the Squamata. 



This order comprises only a single living genus, Hatteria, 

 together with a number of fossil forms. 



ORDER III. CHELONIA. 



Reptilia having the body enclosed in a shell of bony plates, con- 

 sisting of a dorsal carapace and a ventral plastron, partly of 

 dermal, partly of endoskeletal origin. There is usually on the 

 surface an epidermal exoskeleton of horny plates. The vertebrae 

 and ribs of the thoracic region are firmly fused with the bony 

 carapace, into the composition of which they enter. The quad- 

 rate is immovably united with the skull. The nasal apertures in 

 the skull coalesce into one. The limbs are sometimes terminated 

 by clawed digits adapted for terrestrial locomotion, sometimes 

 modified into the shape of flippers. There are no teeth, and the 

 jaws have a horny investment. The lungs are compound sacs. 

 In essentials the heart and brain resemble those of the Squamata. 

 There are no copulatory sacs, but a median penis. 



This order includes the Land Tortoises, Soft Tortoises, River 

 and Mud Tortoises, and the Turtles, besides a number of fossil 

 forms. 



ORDER IV. THEROMORPHA. 



Extinct Reptiles with amphicoelous vertebrae sometimes enclosing 

 remnants of the notochord, with a sacrum composed of from two 

 to six vertebrae, and with ribs having bifid vertebral extremities. 

 There is no sternum. The quadrate is not movable. The limbs 

 are adapted for walking. The pubes and ischia are united. The 

 teeth, which are usually, though not always, present, are highly 

 differentiated and lodged in sockets. 



This order comprises a large number of extinct Reptiles, which 

 are grouped in the four sub-orders, Anomodontia, Placodontia, 

 Pareiosauria, and Theriodontia (Fig. 952). 



ORDER V. CROCODILIA. 



Reptiles in which the dorsal surface, or both dorsal and ventral 

 surfaces, are covered with rows of sculptured bony scutes. Epi- 

 dermal scales are also present. The vertebral centra are either 

 .amphiccelous, flat at each end, or procoelous. The anterior thoracic 



