362 GEOLOGICAL SURVEY OF THE TERRITORIES. 



Lodge-Pole Trail, the other by Dr. Corson at Grizzly Buttes. The 

 specimens belonged to mature animals, and both exhibit the molar 

 series with six teeth as in Lo2)Modon and the Tapir. The last lower 

 molar has a bilobed crown as in the latter animal. 



The reduction in the number of premolars from four to three is proba- 

 bly the least important of the characters distinguishing the genera 

 LopModon, Tapirus, and Hyracliyus. 



Prof. Marsh has described the portion of an upper jaw containing 

 seven teeth, which he refers to a si^ecies under the name of LopModon 

 nanus. The specimen was found at Grizzly Buttes, and probably 

 belongs to the species of Hyracliyus just described. 



LOPHIODON. 



LopModon affinis. LopModon pumilus. 



Some remains, from the Tertiary of Wyoming, described by Prof. 

 Marsh and attributed by him to two species of LopModon under the above 

 names, may, in the discovery of more complete material, prove to belong 

 to the former genus. 



Hyopsodus. 



Among the pachyderms of the age of the Bridger Tertiary formation, 

 there Avere several remarkable for their small size, none now being in 

 existence so diminutive, nor, indeed, at any age, is there any evidence 

 of smaller ones. Most of the fossil remains of these animals which have 

 been submitted to my examination consist of portions of lower jaws 

 with teeth. Portions of lower jaws in many formations appear to be 

 among the most frequent of vertebrate fossils. This is due to the com- 

 paratively firm constitution of the lower jaw, and the readiness with 

 which it becomes detached in the decomposition of an animal lying on 

 a muddy bottom of some body of water. Once detached, it readily 

 becomes imbedded in the mild and enshrined in the future rock. The 

 more bulky head, remaining still longer exposed, is liable to be broken 

 up and its fragments scattered. 



Prof. Hayden, Dr. Carter, and Dr. Corson have collected many frag- 

 ments of lower jaws with teeth, of a small pachyderm, at Black's Fork, 

 Grizzly Buttes, Lodge-Pole Trail, and other localities in the vicinity of 

 Fort Bridger, Wyoming, which I have referred to a genus with the name 

 at the head of this chapter. 



The lower -jaw specimens exhibit a continuous arch of teeth, composed 

 on each side of seven molars, a canine, and, apparently, three incisors. 

 IS^one of the latter are retained in any of the specimens ; and the same 

 may be said of the canine, which is a comparatively small or feebly 

 developed tooth. The first premolar is inserted by a single fang, which 

 is, however, broad and apparently constituted of a connate pair. The 

 other premolars, and the succeeding molars, have each a i)air of fangs. 



As characteristic of the genus, and distinguishing it from other small 

 pachyderms with which it was associated, we may describe especially 

 the first and second true molars of the lower jaw. TLiese have an ob- 

 long crown of nearly uniform width, composed of an outer pair of demi- 

 conoidal lobes, of which the posterior is the larger, and an inner pair of 

 conical lobes, of which the anterior is the larger. The summits of the 

 Quter lobes are crescentoid ; those of the inner ones simply pointed. The 

 contiguous horns of the inner crescentoid summits join the antero- 

 iuternal lobe. The anterior horn of the anterior crescentoid summit 



