xii PHYLUM CHORDATA 445 



most anterior or the most posterior of a considerable range of 

 maxillary teeth. In the Vipers the large poison-fang is 

 capable, owing to the maxilla in which it is fixed being 

 movable on a hinge-joint, of being rotated through a consider- 

 able 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 his mouth to strike its prey. 



In Hatteria 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, 

 represented 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, and each becoming replaced, when 

 worn out, by a successor developed on its inner side. 



In the enteric canal of the Reptiles the principal special 

 features to be noticed are the muscular gizzard-like stomach 

 of the Crocodilia, the presence of a rudimentary caecum at 

 the junction of small and large intestines in most Lacertilia 

 and in the Ophidia, and the presence of numerous large 

 cornified papillae in the oesophagus of the Turtles. 



The Reptiles have all an elongated trachea^ the wall of 



