xii PHYLUM CHORDATA 447 



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

 vipers 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 colu- 

 brine 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, owing to the 

 maxilla in which it is fixed being movable on a hinge -joint, 

 of being rotated through a considerable angle, and moved 

 from a nearly horizontal position, in which it lies along the 

 roof of the mouth, embedded in folds of the mucous mem- 

 brane, 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 pre-maxilla 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 pre-maxillae, 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 



