80 



ON THE HOMOLOGIES AND ORIGIN OF THE 



Tap irus. 



These observations render it highly probable that the selenodont molar is pro- 

 duced by a modification of the antiodont bunodont molar. Also, that the manner 

 of the change has been by constant acceleration of growth of the folds of the 

 tooth upwards and perhaps downwards in its long axis; and an acceleration in the 



lengthening of the crown. 



Fig. 11. 2. Tapirodonts. — This form is so nearly similar to the Palseo- 



theriodont, that any series annectant between the latter and the 

 Bunodonts will render very probable such a connection for the 

 Tapirodonts also. Indeed, it is clear that the same evidence will 

 be sufficient in both cases, since the premolars and last molar of 

 Lophiodon are amoebodont, like Palceotherium. In point of fact, however, the 

 tubercles of the molars of Achoenodon are partly united in transverse pairs, while 

 there are connecting tubercles connecting the opposite cones in some molars of Elo- 

 therium. These structures foreshadow this group as well as the Trichechodont. 



3. Trichecodonts. — Tlie Mastodons and Elephants form a most complete series 

 between this form and the Bunodonts, as has been pointed out by Falconer. In 

 this series, the transverse rows or pairs of tubercles, as well as the ciests, may be 

 few or many. Thus in Elotherium, HaUtkerium, and Trichechus they are few ; in 

 Trachytherium and Dinotherium more numerous; in Mastodon, Stegodon, Tetra- 

 caulodon, and Elephas, most numerous. The tubercles are united into serrated 

 cross-crests in Halitheriwn, the extinct sea cow; in Trachytherium, another fossil 

 ally of the Manati, the tubercles are not united. The succession from Mastodon 



to Elephas may be 

 represented by the 

 accompanying fig- 

 ures. Fig. 1 2 (from 

 Cuvier) is a molar (f; 

 of M. angustidens, I '^^ 

 where, besides the 

 principal tubercles, 

 numerous lesser 

 ones appear. Fig. 13 represents 

 Tetracaulodon ohioticus, in which 

 the opposite tubercles are nearly 

 united into transverse crests. In 

 Dinotherium (Fig. 14) and Elephas 

 (Fig. 15) the union is complete. 

 The relations of these genera have been described 



Fis. 12. 



13. 



XV- 





Fig. 15. 



Fig. 14. 



