318 



COMPARATIVE ANATOMY 



Cholonians, like existing Birds, are provided with horny sheaths 

 to the jaws instead of teeth. The presence of teeth in the embryo 

 of Trionyx, as well as of a vestigial dental lamina in embryos of 

 Chelone and certain Birds, proves, however, that this is only a 

 secondary condition. Moreover, in the Cretaceoiis Birds of 

 N. America (Odontornithes) teeth were present, and were either 

 situated in definite alveoli (Ichthyornis), or simply in grooves 



B 



FIG. 233. 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.) 



GO, poison-canal ; Gz, poison-fang ; PH. pulp-cavity ; Hz, reserve fangs. 



(Hesperornis). The premaxilla? were toothless, and seem to have 

 possessed a horny beak. The single-pointed teeth of Archaeopteryx 

 were probably situated in alveoli. 



Mammals. 



In Mammals, a shortening of the jaws, which results in an 

 increase in their power and a diminution in the number of teeth, 

 has taken place along with a higher differentiation of the individual 

 tooth and a reduction in the number of tooth-generations. The 

 heterodont condition which characterises Mammals, as a Avhole, 

 has arisen from a more primitive homodont condition, and the 

 teeth have become adapted in various ways in correspondence with 



