PHYLUM CHORDATA 



355 



there is no true symphysis, as their extremities remain cartilaginous 

 A hypo-ischium is not present. In the tarsus (Fig° 998) 

 there are two proximal bones— an astragalO-scaphoid° a,ni a 

 calcaneum—the latter having a prominent calcaneal process, 

 and two distal tarsal bones, together with a thin plate of 

 cartilage supporting the first and second metatarsals. The 



missing fifth digit is re- 

 presented by a rudimentary 

 metatarsal. 

 vjBH Digestive Organs. — The 



form and arrangement of the 



Fig. OSS.— Tarsus of Crocodile (right side) 

 from above. F. fibula ; 2'. tibia ; t i. c. tho 

 astragalus, formed of the united tibiale, 

 intermedium and centrale ; /. fibulare 

 (calcaneum) ; 1—3, united first, second 

 and third distal tarsals ; 4, fourth t.arsal ; 

 I— I V, first to fourth metatarsals ; P?, fifth 

 distal tarsal and fifth metatarsal. (From 

 Wiedersheim's Comparative Anatoriiy.) 



teeth already described in the 

 account of Lacerta prevail in 

 the majority of Lizards. In 

 some of them the palatine 

 teeth are absent. The teeth 

 are sometimes fixed by their 

 bases to the summit of the 

 ridge of the jaw {acrodont 

 forms), sometimes fixed by their sides to the lateral surface of 

 the ridge (pleurodont) ; they are never embedded in sockets 

 in any recent form. A Mexican .Lizard, Heloderma, differs 

 from all the rest in having teeth which are grooved for the 

 ducts of poison-glands. In the Snakes (Figs. 988, 989) teeth 

 are rarely developed on the premaxillse, but are present on the 

 maxillae, palatines and pterygoids, as well as the dentary of 

 the mandible. They may be of the same character through- 

 out, solid, elongated sharp-pointed teeth, which are usually 



Fig. 997.— Pelvis of young Alligator, ventral 

 aspect. B, fibrous band passing between the 

 pubic and ischiadic symphyses ; BR. last pair 

 of abdominal ribs ; F. obturator foramen ; 

 G. acetabulum ; /(. ilium ; /». ischium ; M. 

 fibrous membrane between the anterior ends 

 of the two innominate bones and the last pair 

 of abdominal ribs ; P. pubis ; Sy. ischiadic 

 aymDhysis ; 1, II, first and second sacral 

 vertebrai. (From Wiedersheim's Cmnparative 

 Anatomy.') 



