xin PHYLUM CHORDATA 345 



Digestive Organs. The form and arrangement of the 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. 999, 

 1000) teeth are rarely developed on the premaxillee, but are present 

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

 the mandible. They may be of the same character throughout, 

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

 recurved, so that they have the character of sharp hooks, their 

 function being to hold the prey and prevent it slipping from 

 the mouth while being swallowed, not to masticate it. Non- 

 venomous Snakes possess only teeth of this character. In the 

 venomous Snakes more or fewer of the maxillary teeth assume 

 the character of poison-fangs. These are usually much larger 

 than the ordinary teeth and either grooved or perforated by a 

 canal for the passage of the duct of the poison-gland. In the 

 Vipers (Fig. 1000) there is a single large curved poison-fang with 

 small reserve-fangs at its base, these being the only teeth borne 

 by the maxilla, which is very short ; in the venomous Colubrine 

 Snakes the poison-fangs are either the most anterior or the most 

 posterior of a considerable range of maxillary teeth. In the Vipers 

 the large poison-fang is capable of being rotated through a con- 

 siderable angle, and moved from a nearly horizontal position, in 

 which it lies along the roof of the mouth embedded in folds of the 

 mucous membrane, to a nearly vertical one, when the Snake opens 

 its mouth to strike its prey. The rotation of the maxilla is brought 

 about by the backward or forward movement of the pterygoid 

 with the palatine and transverse. In Sphenodon (Fig. 1001) 

 there are pointed, triangular, laterally-compressed teeth, arranged 

 in two parallel rows, one along the maxilla, the other along the 

 palatine. The teeth of the lower jaw, which are of similar character, 

 bite in between these two upper rows, all the rows becoming worn 

 down in the adult in such a way as to form continuous ridges. 

 Each premaxilla bears a prominent, chisel-shaped incisor, repre- 

 sented in the young animal by two pointed teeth. In the young 

 Hatteria a tooth has been found on each vomer a condition 

 exceptional among Reptiles. In the Chelonia, teeth are entirely 

 absent, the jaws being invested in a horny layer in such a way 

 as to form a structure like a Bird's beak. The Crocodilia have 

 numerous teeth which are confined to the premaxillae, the maxillae, 

 and the dentary. They are large, conical, hollow teeth devoid 

 of roots, each lodged in its socket or alveolus (thecodont), and each 



