-'*'"■ -1 COPE ON EXTINCT RIIINOCERID.E. 233 



etc., wliicli I have named the palteotheriodont, took its origin from the 

 type vdiich is intermediate between it and the bnnodont, viz, the sym- 

 borodont, as I have pointed ont 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 LopModontidcc of 

 the genns Colonoceras. The same character has been observed by Marsh 

 in species of the Lower Miocene, which probably belong to the true 

 EJiinocendce, and which he has called Dicer atlierium. This genus ap- 

 pears to have terminated the line exhibiting this structure, and the 

 family in ISTorth America remained without horn. As we have seen, the 

 types possessing the median horn arose in Europe, in the Ceratorhimia 

 scJileiennacheri 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 JSTorth America with the ad- 

 vance of geologic time. Thus, their probable ancestors of the genus 

 Eyracliyus were the least of all. The Aceratheria of the Wliite Eiver 

 formation were larger ; the oldest, A mite, being the smallest ; and the 

 later A truquianum of Oregon, being the largest. The species of the 

 Loup Eiver or Upper Miocene formation were all larger, and were nearly 

 equal to the large existing species. 



The characters of the American genera and their allies, Hyracodon 

 and AncMsodon, are pointed out in the following pages. 



AXCHISODO]^" Cope. 



American Naturalist, 1879, 270, April ; published March 26, 1879. 



This genus is represented by maxillary teeth only, so that the charac- 

 ters of the incisor teeth and inferior molars are unknown. There are 

 four premolars and three true molars. The true molars are constructed 

 much as in Lopliiodon. They have two external confluent crescents, each 

 of which gives origin at its anterior part to a transverse crest, which is 

 directed more or less obliquely backwards. The anterior part of the ante- 

 rior external crescent is well developed, and not shortened as in Acerafhe- 

 rium and Bliinocerus, and terminates in a low cusp. The external wall of 

 the last true molar is continued posterior to the posterior transverse crest 

 as in tapiroid types. The third and fourth premolars resemble the true 

 molars. The second i^remolar diifers from the others in a relatively 

 greater elongation of the external portion, which presents three cusps 

 instead of two. Of these one is opposite to the origin of each cross-crest, 

 and the third is between them. The anterior part of the anterior exter- 

 nal crescent is produced, as is also the posterior part of the posterior. 



In the milk dentition, the teeth which occupy the i)ositiou of the tliird 

 and fourth permanent premolars are first protruded, and afterwards the 

 second deciduous premolar, and later the first true molar. 



*The Homologies aud Origin of the Molar Teeth of 3IaiJjmalia, etc. Journal 

 Academy Nat. Sciences, Philada., 1874, pp. 13-14. 

 Bull. V, 2 G 



