216 



COMPARATIVE ANATOMY. 



at a short distance from the apex of the tooth (see the course of 

 the arrow in Fig. 176, A). 



Between the ordinary teeth of Snakes and the poison-fangs with closed 

 canals, there are numerous intermediate forms in which certain of the teeth are 

 simply grooved along their anterior side. A similar condition is also seen in 

 the teeth of the lower jaw of a poisonous Mexican Lizard (Heloderma). 

 (Comp. p. 222.) 



Chelonians, like Birds, are provided with horny sheaths to the jaws in- 

 stead of teeth. The presence of teeth in embryos of Trionyx, however, proves 

 that this is only a secondary condition. 



The teeth of the fossil Birds of America (Odontornithes) were either 

 situated in definite alveoli (I ch thy or nis), or simply in grooves (Hesperornis). 

 The premaxillae were toothless, and seem to have possessed a horny beak. 

 The single-pointed smooth teeth of Archseopteryx were 1 mm. long, and all of 

 similar size and form. Most probably they were situated in alveoli. All 

 existing Birds, as well as those of the Tertiary and Post-Tertiary strata, are 

 toothless. 



FIG. 176. FIGURES OF THE POISON-FANGS OF A VIPERINE SNAKE. 



A, skull of Rattlesnake ; B, transverse section through about the middle of the 

 poison-fang of Vipera ammodytes ; C, transverse section through the poison-fang 

 of Vipera ammodytes near its distal end. (B and C after Leydig.) 



Gz, poison-fang ; Rz, reserve fangs ; GC, poison-canal ; PIT, pulp-cavity. 



Mammals. The differentiation of the dentition here reaches 

 its extreme limit, corresponding with the manner in which the food 

 is taken in and masticated. As already mentioned, incisor, 

 canine, and grinding teeth (premolars and molars) can 

 as a general rule be distinguished. These are all embedded in 



