360 GEOLOGICAL SUEVEY OF THE TEEEITOEIES. 



that had they been found isolated I should have been misled and con- 

 , sidered them as such. These teeth curve from the jaw parallel with 

 each other as in the gnawers, but they are separated from each other 

 by an interval sufficiently large to be occupied by a pair of small teeth. 

 The large incisors do not extend so far back in their sockets as in the 

 rodents, and in this respect i^resent a condition more like that in the 

 Hog and Peccary. 



The constitution of the large incisors of Trogosus resembles that in 

 the gnawers, and as in these they apparently were provided with per- 

 manent dental pulps, so that they continued their growth and protru- 

 sion as they were worn away at the gnawing extremity. The fore part 

 of these teeth is more convex than in rodents, and the enamel extends 

 to a greater depth at the sides. They were also worn away in a some- 

 what different manner. In rodents the opposing incisors of the upper 

 and lower jaw are worn off in a sloping manner from the enamel cutting- 

 edge backward toward the sockets of the teeth. In Trogosus the worn 

 slope of the lower incisors is not only directed backward and dow^nward 

 but also outward. This would indicate a divergence of the ux)per 

 incisors, which no doubt hold a position, when the jaws are closed, 

 exterior to that of the lower incisors. 



Canine teeth api^ear not to have existed, at least none of any size or 

 of importance as efficient organs in the dental series. In this respect 

 Trogosus is like the Mastodon, the Elei3hant, the Ehinoceros, and the 

 Hyrax. 



Trogosus castoridens. 



The species named from its incisors, recalling to mind the powerful 

 cutting teeth of the Beaver, was a much larger and proportionately more 

 robust animal than this. It was about the size of the White-lipped 

 Peccary of Brazil. Extent of the series of six lower molars nearly 4 

 inches. Extent of the series of true molars, 2^ inches. Depth of large 

 incisors, fore and aft, 10 lines ; breadth, half an inch. 



Trogosus vetulus. 



A second and smaller species is indicated by a portion of an incisor 

 tooth, likewise discovered by Dr. Carter in the vicinity of Fort Bridger. 

 The specimen, though a mere fragment, is quite characteristic, as it 

 preserves the peculiar form and also the mode of wearing of the cutting 

 extremity. The species was about two-thirds the size of the former one. 

 The diameter of the incisor from before backward is 7 lines; transversely, 

 4 lines. 



Hyeachyus. 



Among the most abundant mammalian remains of the Bridger Ter- 

 tiary formation are those of a genus of odd-toed pachyderms, allied to 

 the living Tapir, to which the above name has been given. The genus 

 was first indicated by some imperfect remains obtained on Smith's Fork 

 and Black's Fork of Green Eiver, Wyoming, during Prof. Hay den's 

 exploration of 1870. The specimens are noticed in Prof. Hayden's 

 Preliminary Eeport of the United States Geological Survey of Wyoming, 

 &c., 1871, p. 359. Since then, during the last summer. Dr. J. Van A. 

 Carter discovered a number of more perfect specimens, characteristic of 

 the genus, on Henry's Fork of Green Eiver and at Bridger Butte, in 

 the vicinity of Fort Bridger, Wyoming. Dr. Joseph K. Corson, U. S. A., 

 has also collected additional material at Grizzly Buttes, Wyoming. 



From the many specimens, consisting of portions of jaws with most 

 of the teeth, together with other portions of the ^skeleton, which have 



