168 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Vol.YI. 



enlarged size of the superior canine teeth ; (3) in the diminished size of 

 the inferior canine teeth ; (4) in the conic form of the crowns of the in- 

 cisors ; (5) in the addition of a cutting lobe to the anterior base of th6 

 superior sectorial tooth ; (6) in the obliteration of the inner tubercle of 

 the lower sectorial, and (7) in the extinction of the heel of the same ; 

 (8) in the development of an inferior flange and latero-anterior angle of 

 the front of the ramus of the lower jaw; (9) in the development of cut- 

 ting lobes on the posterior borders of the larger premolar teeth. 



(1) The reduction in the number of molar teeth. The dental formula 

 of Frocelur us is that of some Viverridwand Canidce, and the reduction from 

 this point to the end of the series is obvious. In JEtismilus, as in Smilodon, 

 the number of molars is less by one in the inferior series than in Lynx 

 and N'eofelis, where the formula is the smallest known among Felidce 

 proper, viz : f \. (2) The enlarged size of the superior canine teeth. In 

 Procelurus and Pseudwlurus the canines of both jaws are developed, 

 as in recent Felidce. In Archcelurus the superior is the larger, but 

 does not, relatively to the molars, exceed that of Felis. It is rather 

 compressed in form and has a sharp cutting edge posteriorly. In 

 Nimravus the superior canine begins to have the enlarged size of the 

 saber-tooths, but its form is i)eculiar in the N. gompJiodus, being spike- 

 shaped rather than saber-shaped. We find the true saber shape first in 

 Ifinictis, where it is compressed, and with a denticulate cutting edge on 

 both front and rear. In Pogonodon it has reached a very large size, and 

 it does not display much increase in this respect until we reach the 

 lasu genus of the series, Fusmilus, where its proportions are enormous, 

 almost as large as in the feline genus iSmilodon, where they ajjpear to 

 have been an inconvenience to the animal. (3) The diminished size of 

 the inferior canines becomes evident in the lower genera of the third 

 division (supra) of the Nimravidm, but is most decided in the highest 

 genera, HoplopJioneus and Fusmilus. (4) The incisor teeth have the usual 

 obspatulate or obovate outline in the genera of the first and second di- 

 visions of the family, including Nimravus. They are conic in the true 

 saber-tooths with flared lower jaw, beginning with Dinictis and ending 

 with Fusmilus. (5, 6, and 7) The structure of the sectorials. The pres- 

 ence of a heel and an inner tubercle of the lower sectorial are well-known 

 characters of a majority of the Carnivora. In only the most highly or- 

 ganized genera are they wanting, and among them are included all those 

 of the Felidce that still exist. In the Nimravidce the inferior genera 

 have both in a reduced degree, and they soon disappear as we ascend 

 the scale. Thus, the inner tubercle is only j)resent in the species of 

 Procelurus, Finictis, and HoplopJioneus. The heel, on the other hand, re- 

 mains throughout the entire family. The anterior basal lobe of the 

 superior sectorial has the same history, its absence being characteristic 

 of the inferior Carnivora, and of all the genera of Nimravidce except 

 Hoploplioneus, where it is rudimental. It is well developed in Frepano- 

 don as in recent Felidce, and is sometimes double in Smilodon. (8) The 



