354 Wortman — Studies of Eocene Mammalia in the 



more complex and perfectly molariform character of the 

 fourth premolar. In the Bridger, from which the type of the 

 family was derived, there are at least four well-marked species 

 now known. The characters of the genus, as understood 

 almost exclusively from the dentition, are as follows : There is 

 but a single pair of incisors in the lower jaw and presumably 

 a like number in the upper jaw ; the enamel is not limited to 

 the anterior face of the tooth, as in the Rodentia, but completely 

 invests the crown, and the teeth are not of continuous growth ; 

 a small canine and two premolars are present, or no canine and 

 three premolars, according to the way in which we interpret 

 the first small tooth behind the enlarged incisor to be a pre- 

 molar or canine ; except, perhaps, in one species, the fourth pre- 

 molar above and below is completely molariform ; the superior 

 molars are tritubercular in structure, with a faint beginning of 

 a fourth cusp and a slightly developed mesostyle, which be- 

 comes stronger in the later species ; the two rami of the lower 

 jaws are not coossitied. 



I know of no remains of other parts of the skeleton that 

 with certainty can be referred to any species of the genus. I 

 have seen, however, some skeletal fragments which I strongly 

 suspect belong to a species of this genus, but I lack the evi- 

 dence to make the necessary connections. 



Microsyo2)s elegans Marsh. 



Limnofherium elegans Marsh, this Journal, January, 1871, p. 12; Micro- 

 syops gracilis (in part) Leidy, Proc. Acad. Nat. Sci. Phila., 1872, p. 20; 

 Mesacodon speciosus Marsh, this Journal, September, 1872, p. 205 ; Palceaco- 

 don veriis Leidy, Proc. Acad. Nat. Sci. Phila., 1872, p. 20; Microsyops 

 elegans Cope, Tertiary Vertebrata, 1883, p. 217 ; Microsyops gracilis Osborn, 

 American Eocene Primates, Bull. Amer. Mus. Nat. Hist., 1902, p. 210. 



Description of the Type. — The type of this genus and spe- 

 cies consists of a fragment of a left mandibular ramus bearing 

 the first and second molars, together with the fourth premolar, 

 and the roots of the last molar. The anterior and posterior 

 parts of the jaw are not preserved, so that it is impossible to 

 determine the full dentition. The molar crowns may be de- 

 scribed as consisting of an anterior, tricuspidate, elevated por- 

 tion, usually termed the trigon, and a posterior, wider, less 

 elevated part, or heel. The three cusps of the trigon are 

 conical, and are placed in the form of a more or less per- 

 fect equilateral triangle, with the apex directed forward. Of 

 these, the anterior is much the smallest of the three, the two 

 posterior cusps being subequal in size and standing nearly oppo- 

 site each other. The heel is considerably wider than the ante- 

 rior, or trigonal, part of the crown and bears three distinct 

 cusps enclosing a basin. Of these, one is external, one inter- 

 nal, and one posterior. The external cusp is the largest and 



