KINE TO GENESIS. 



329 



Of hypertrophy we have two types : the first repre- 

 sented by the Ghres, Muhituberculata, Tillodonta and 

 their ancestors; and second, by the Proboscidia, the 

 narwhal and certain Sirenia. As the uses of the inci- 

 sors present two types corresponding with their struc- 

 ture, we have ground for believing the uses in question 

 to have been the efficient agent in producing the lat- 

 ter. Esthonyx furnishes us with an example (Fig. 96) 

 where all the incisors are present in the lower jaw, and 





"A 





.'-^i. <> 



ii 



Fig. 97. — Psittacotherium niultifragum Cope, mandibular ramus, one-half 

 natural size ; a, profile ; l\ from above. 



where the function of one pair of them (the second) 

 has evidently been partially rodent in character; that 

 is, it has served as a scraper and gouger of food sub- 

 stances. Persistent use has apparently developed the 

 size of this pair of teeth, until we find in Psittacothe- 

 rium (Fig. 97) they have reached a greater efficiency, 

 and that the external incisors of the lower jaw have 

 disappeared. This disappearance can be accounted 

 for on the ground of disuse, a retirement from service 

 due to position, and the increased growth of incisor 



