REPTILIA. 209 



"When the exoskeleton is complete (as in Teleosaurus and 

 in Caiman), it consists of transverse rows of quadrate bony 

 plates disposed so as to form a distinct dorsal and ventral 

 shield, which are separated by soft skin in the region of the 

 trunk, but become confluent in the tail. All the scutes of 

 one row are united by suture, and successive rows usually 

 movably overlap one another. 



The order Crocodilia is divided by Owen into three sub- 

 orders, termed Proccelia, Amphiccelia, and Opisthoccelia, accord- 

 ing as the dorsal vertebrae are concave in front, concave at 

 both ends, or concave behind.-^ The sub-order Froccelia com- 

 prises all the Living forms — namely, the Crocodiles proper, 

 the Alligators, and the Gavials. The first of these have the 

 fourth tooth in the lower jaw (fig. 559) larger than the 

 others, forming a canine tooth, which is received into a notch 

 excavated in the alveolar border of the upper jaw, so that 

 it is visible externally when the mouth is closed. In the 

 Alligators (fig. 560), the fourth tooth in the lower jaw forms 

 a canine which is received into a pit in the palatal surface of 

 the upper jaw, where it is completely concealed when the 

 mouth is shut. In the Gavials the snout is greatly prolonged, 



^ The following more elaborate classification of the recent and extinct Croco- 

 dilia has been proposed by Pi-ofessor Huxley : — 



I. Parasuchia, with no bony plates of the pterygoid or palatine bones to 

 prolong the nasal passages; the Eustachian passages enclosed by bone; the 

 centra of the yertebrse amphicoelian ; the coracoid short and rounded ; the ala 

 of the ilium high, and its acetabular margin entire; and the ischium short 

 dorso-ventrally and elongated longitudinally, with its acetabular portion re- 

 sembling that of a Lizard. — Genera: Stagonolepis, Belodon. 



II. Mesosuchia, with bony plates of the palatine bones prolonging the 

 nasal passages, and giving rise to secondary posterior nares ; a middle Eus- 

 tachian canal included between the basioccipital and basisphenoid, and the 

 lateral canals represented only by grooves ; vertebral centra amphicoelian ; 

 coracoid elongated ; ala of the ilium lower than in the preceding, higher than 

 in the next sub-order, its acetabular margin nearly straight ; ischium more 

 elongated dorso - ventrally than in the preceding group, with its acetabular 

 margin deeply notched. — Genera : Steneosaurus, Pelagosmvrus, Teleosaurus, 

 Teleidosaurus, Metriorhy7ichus (Goniojjholis ? Fholidosaurus ?). 



III. EcsucHiA, with both pterygoid and palatine bones giving off plates 

 which prolong the nasal passages ; vertebral centra mostly procoelous ; coracoid 

 elongated ; ala of the ilium very low in front, its acetabular margin deeply 

 notched ; ischium elongated dorso-ventrally, with its articular margin deeply 

 excavated. — Genera: Thoracosaurus, Holops, and the recent forms. 



VOL, II. 



