96 Annals of the Carnegie Museum. 



The mandible associated with the type of Merycochcerus proprius is 

 very different in form from the type of Pronomotherium laticeps which is 

 not quite so old an individual. The depth of the jaws is nearly the 

 same at the chin, but in the last named specimen the lower border of the 

 jaw begins to drop beneath M^-, and under the posterior portion of 

 Mg it becomes exceedingly deep. The incisors and canine are far less 

 robust. The proportional length of the premolar to the molar series 

 is apparently somewhat less. The molars and premolars have about 

 the same pattern, but in the present type are more hypsodont. In 

 nearly all respects the specimen of Pronomotherium has a more ad- 

 vanced or specialized appearance. 



Comparison with Merycochcerus? rusticus. 



The specimen of Pronomotherium is apparently much more like 

 Merycochcerus ( ?) rusticus than Merycochcerus proprius. The symphysis 

 of the premaxillanes, the concavities of the sides of the face, the way 

 the infraorbital foramen opens, the sudden widening of the skull at 

 the anterior portion of the zygomatic arches, the reduction in the size 

 of the incisors, and the form of the chin and other portions of the 

 mandible are much the same in both, yet there are slight differences 

 in all of these. 



Pronomotherium laticeps is considerably larger than Merycochcerus (?) 

 rusticus, the anterior palatine foramina are smaller ; the shelf at the 

 bottom of the facial concavity — the top of the malo-maxillary ridge 

 — is flatter and more horizontal ; the malo-maxillary ridge is nar- 

 rower and more angulate, not broadly and evenly convex as in Mery- 

 cochcerus (?) rusticus. Premolars one and two do not incline backward 

 and become much more worn on the posterior edges as in Merychyus. 

 The fourth premolar has a larger inner cingulum, and molars one and 

 two have more prominent buttresses. 



It may be that Merycochcerus rusticus belongs in the same genus as 

 Pronomotherium laticeps, but it is still very doubtful, as the type of 

 the former is so incomplete, and Dr. Matthew refers the specimens 

 from Colorado 20 to this species, with some doubt. 



I have made detailed comparisons of Pronomotherium laticeps with 

 specimens obtained by O. A. Peterson in the Loup Fork (Upper 

 Monroe Creek or Harrison beds) of Nebraska. These are much like 

 the specimens which Dr. Matthew refers to Merycochcerus proprius but 



20 "Extinct Mammals from Colorado," p. 412. 



