472 THE MAMMALIA OF THE UINTA FORMATION. 



Bridger species only in size. Since the first account of this sjDecies was written, we 

 have found that the order of the teeth was there incoi-rectly given ; what was there 

 described as the second premolar being probably the third molar. The most anterior 

 premolar of the lower series pi-eserved in the specimen is probably the second (see 

 PI. X, Fig. 9). This is a relatively small tooth, implanted by two fangs and with 

 a low, rather elongated crown, which consists of a median cone and faintly separated 

 anterioi' basal tubercles ; this tooth is therefore somewhat differently constituted from 

 the second lower premolar of M. obtusidens, in which the anterior tubercle is not in- 

 dicated at all, and the posterior one scarcely so. The fourth premolar has nearly the 

 shape and size of the true molars, consisting of a high and massive, backwardly 

 directed principal cusp, which is separated by a deep and narrow cleft from the lai-ge 

 trenchant heel. The latter is as long from before backwards as the main cusp and is 

 strongly convex on the outer side and concave on the inner. This tooth diftei-s from 

 the fourth premolar of 3f. oltusidens only in the absence of the anterior basal cusp. 

 The first true molar has the same construction as the tooth just described, but is 

 larger, and the main cusp is remarkably massive; there is an indication of the anterior 

 basal cusp, but it is much less distinctly marked than in the Bridger species. This 

 tooth is the largest of the series and considerably exceeds in size the lower sectorial 

 of a full-grown lion. The second molar difters from the first only in being a little 

 smaller and in the still greater reduction of the anterior basal cusjjs. The develop- 

 ment of these cusps varies greatly in the ditferent genera of the family ; in Dissacus 

 of the Puerco they are fairly well developed; in the Wasatch genus Pacliycuna they ai-e 

 greatly increased in size, becoming as large as the heel, which gives the inferior 

 molai's a trifid appearance when seen from the side. In the Bridger species of Meso- 

 nyx these cusps become much smaller, being greatly exceeded in size by the talon, 

 while in M. uiniensis they have almost disappeared, entirely so in fact, from the 

 fourth premolar. The third molar is very small, and is even more reduced in size 

 than in M. obtusidens, which reduction chiefly affects the talon ; in the Bridger spe- 

 cies the talon is of good size, but in the specimen before us the heel is a mere rudi- 

 ment. The main cusp is also more compiessed and pointed. The third molar is very 

 differently developed in the various genera of the family ; in Disaacus and Pacliyieaa 

 iiT^B is nearly or quite as large as m7^ ; in Mesonyx obtusidens it is distinctly smaller, 

 the proportions being 7 : 9, and in M. tiinfensis it is still smaller, as 3: 5. This reduc- 

 tion in 3Iesonyx is, of course, correlated with the loss of the last upper molar, and the 

 pi'ojwrtions in the Uinta species would seem to indicate that the second upper molar 

 was undergoing a similar reduction. 



It has been geneially assumed that the molar teeth of the Mesonychidoe are of 



