IS 



ON THE HOMOLOGIES AND ORIGIN OF THE 



tions are very obvious, so much so as to lead to the belief that the several subdivisions 

 of the Lophodonts represent modifications of corresponding types of Bunodonts, 

 and that the two are partially "homologous groups." Both present corresponding 

 Amoebodont and Antiodont types; as an example of the former kind of Bunodont, 

 the mandibular molars of the genus Ilyoipsodus may be cited ; of the latter kind, 

 the same of the genus Achcenodon, both the earliest, or Eocene genera. It remains 

 to indicate the intermediate forms, if any there be, which give color to the sup- 

 position that the various divisions of Lophodonts have descended from Bunodont 

 predecessors. Here, then, I mention a fact of prime importance; i. e., that in 

 America at least, no Selenodonts are known from formations of older age than 

 Miocene ; while the greatest development of Bunodonts is in the beds of the next 

 older epoch, the Eocene. 



The special forms of Lophodonts may be separately considered as follows :— 

 First, as to the opposite and alternate types, or the Antiodont and Amoebodont. 

 They pass into each other by many intermediate conditions among the Bunodonts, 

 as in Notharctus, Limnotherium, etc. of the Wyoming Eocene. There is reason, 

 also, to believe that this has been the case with some of the Lophodonts after they 

 had -left the bunodont stage behind. Thus Equus is an antiodont as to its upper 

 molars, but has been probably derived from Palceotheriodont ancestors, which are 

 amoebodont ; this is rendered especially probable by the fact that the mandibular 

 teeth are of the amoebodont division (hippodont). It is also highly probable that 

 the antiodont genus Tapirus, though so near to Palceotherimi, was derived from an 

 antiodont Bunodont. Hence, while the discrimination between opposite and alter- 

 nate types is in some cases most radical, in others its importance is but slight. 



I. Antiodonts : Bunodont type Achcetiodon. (Fig. 5.) 



1. Selenodont type approximated by the 

 bunodont Hippopotamus, where the tubercles 



Fig. 5. 



Acha'iiodon. 



are compressed, thus: the intervening valleys 

 are deepened, and the cusps wear readily into 



Fig, 6. 



Hippopotamus. 



