216 



< < >-M PAR ATI V K A X AT( ).M Y. 



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

 the arrow in Fig. 176, A). 



Bet\vrrn the ordinary teeth of Snakes and the poison-fangs with closed 



anal.-, tln-iv an- numerous in! erinei 1 i;it c forms in which certain of the teeth are 



-iin]il\ grooved along their anterior side, A similar condition is also seen in 



tin- teeth of the lower jaw of a poisonous Mexican Lizard (Helodermu). 



Comp. p. 222.) 



Chelo],iaiis, like Birds, are provided with horny sheaths to the jaws in- 

 stt-ud dt teeth. The presence of teeth in embryos of Trionyx, however, proves 

 that tliis is only a secondary condition. 



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

 .-i mated in definite alveoli (Ich thy ornis), or simply in grooves (Hesperornis). 

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

 The Millie-pointed smooth teeth of Aivh;iopteryx were 1 mm. long, and all of 

 -imilar si/e 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. 



fl/ 



FlG. 17''-. Fl'.l RES "I MM. I'olsoN-FANUS OK A YlPKUINK SNAKK. 



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

 |Miisi>n-faii<,' of Vipera ammodytes ; C, transverse section through the poison-fang 

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



<!:, poison-fan^' ; //-., re-urve fan^s ; GO, poison-canal ; PIf, 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 



aa 



genera] rule lie distinguished. These are all embedded in 



