PEEISSODACTYLA. 693 



<3iffers in the primitive condition of the premolars above, which, as in the 

 Lophiodontidce, differ from the molars in their greater simplicity. Thus it is 

 probable that tapiroids, probably Lophiodontidoe, gave origin to the Bkino- 

 •ceridce, as Marsh has suggested. And it is further altogether probable that 

 the general type of dentition presented by the Rhinoceridce, Lojjhiodontidce, 

 etc., which I have named the palseotheriodont, took its origin from the type 

 -which is intermediate between it and the bunodont, viz, the symborodont, 

 iis I have pointed out in an essay on this subject. 



The first appearance of dermal horns was apparently in a pair placed 

 transversely on the nasal bones, in species of Eocene Lophiodontidce of the 

 genus Colonoceras. The same character has been observed by Marsh in 

 species of the Lower Miocene, which probably belong to the true Rhino- 

 ceridce, and which he has called Diceratherium. This genus appears to 

 liave terminated the line exhibiting this structure, and the family in North 

 America remained without horn. As we have seen, the types possessing 

 the median horn arose in Europe, in the Ceratorhinus schleiermacheri of the 

 Middle Miocene, and still survives. 



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

 the species of this 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 Coenopoda of the White River formation were 

 larger; the oldest, C. mite, being the smallest. The Aphelopes of the Loup 

 River or Upper Miocene formation were all larger, and were nearly equal 

 to the large existing species. 



TAPIRID^. 



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

 matotherium, appears in the Middle Eocene (Bridger), and Tapirus is first 

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

 family belong to Tapirus L., and Elasmognatlms (Gill). A small species, 

 the Tapirulus hyracinus Grerv., 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 determine 

 its position. In America a genus which has not yet been distinguished 

 from Tapirus is found in the Miocenes. 



