1881. J ^"5 [Cope. 



It may be observed in conclusion that a successive increase of size in the 

 species of tliis line has taken place in North America with the advance of 

 geologic time. Thus, their probable ancestors of the genus Triplopus were 

 the least of all. The Ccenopoda of the "White River formation were larger ; 

 the oldest G. mite, being the smallest. The Aplielopes of the Loup River 

 or Upper Miocene formation were all larger, and were nearly equal to the 

 large existing species. 



TAPIRIDJE. 



The genera of this family are not numerous as yet. The oldest, Listri- 

 odon, appears in the Middle Miocene (Gers, France), and Tapirus is first 

 found in the Upper Miocene (Epplesheim). The recent species of the fam- 

 ily belong to Tapirus L., and Elasmognathus (Gill). A small species, 

 the Tapir^dus Jiyracinus Gerv., is from a bed at Perreal, France, which 

 Pictet has identified with the gypsum of Paris (Oligocene). It is some- 

 times referred to this family, but is not sufficiently well known to deter- 

 mine its position. In America, Listriodon, or a genus which has not yet 

 been distinguished from it, is found in the Miocenes. 



The three genera are distinguished as follows : 

 Three anterior premolars different from fourth premolar 



and true molars ; last inferior molar with heel. Listriodon. 



One superior premolar different from true molars ; no heel 



of third inferior molar ; nasal septum cartilaginous. . . . Tapirus. 



Like Tapirus, but nasal septum osseous Elasmognathus. 



CHALICOTHERIID^. 



Gill ; Cope, American Naturalist, 1881, p. 340. 



This family had numerous representatives during Eocene time, and a 

 few species of CTialicotTierium extended into Miocene time. The bound- 

 aries which separate the family from the LopModontidm on the one hand 

 and the Menodontidce on the other, are not always easy to determine. 

 From the former the symmetrically developed external Vs of the superior 

 molars, and the double Vs of the inferior molars distinguish it. Yet in Bhaga- 

 therium the external Vs are not so well distinguished as in other Chalico- 

 iheriidw; and 'u\ PropalcBotherium, the anterior cingular cusp produces a 

 part of the assymmetry found in the Lophiodontidce. The character of the 

 double inner cusps of the superior premolars, which distinguish the 

 Menodontidce., is only applicable to the last premolar in Biplacodon of the 

 latter, while a trace of the additional cusp of this tooth is found in the 

 Chalicotheroid NestoritJterium. 



In using the following table it must be borne in mind that the number of 

 the toes has been determined in a very few of the genera. Should any 

 of them prove to have but three digits on the anterior foot, such genera 

 must be referred to a new family intermediate between this one and the 

 PaloBotheriidce. 



PROC. AMER. PHILOS. SOC. XIX. 108. 2x. PRINTED MAY 16, 1881. 



