MOLAR TEETH OF THE MAMMALIA EDUCABILIA. 255 



Fig. 24. — Merychyus major. 



The writer nearly contemporaneously * recalled the obserya- 

 tions of Leidy that the teeth of the Oreodont ^' Merycliyus are 

 more prismatic, have larger crowns atid shorter roots, approaching 

 the sheep, as Oreodon does the deer." Now Oreodon is Miocene, 

 and MerycJiyus Pliocene. It was then ob- 

 seryed : *' This phenomenon suggests an ex- 

 planation on the score of adajjtation which 

 the other cases do not. The existence dur- 

 ing the later period of a [hardjer material 

 of diet would increase the rapidity of wear- 

 ing of the crown of the tooth, and require a 

 longer crown and greater rapidity of j)rotru- 

 sion. This necessitates a diminution of the 

 basal shoulder and shortening of the roots, producing the prismatic 

 form aforesaid." 



These observations render it highly probable that the seleno- 

 dont molar is produced by a modification of the antiodont buno- 

 dont molar. Also, that the manner of the change has been by 

 constant acceleration of growth of the folds of the tooth upward 

 and i^erhaps downward in its long axis ; and an acceleration in the 

 lengthening of the crown. 



2. Tapir odonts. — This form is so nearly similar to the Palaeo- 

 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 pre- 

 molars and last molar of Lopliiodon are 

 amoebodont, like Palceotherium. In point 

 of fact, however, the tubercles of the molars 

 of Achcenodon are partly united in transverse 

 pairs, while there are intermediate tubercles 

 connecting the opposite cones in some molars 



of Elotlierium. These structures foreshadow this group as well 



as the Trichechodont. 



3. Trichecodonts. — The J[/«.«;^Of7o^5 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 crests, may be few or many. Thus 



Fig. 25. — Tapirus. 



*" Proceedings of the Academy of Natural Sciences of Philadelphia," 1868, 

 p. 2Y4. " Origin of Genera," p. 44. 



