186 Annals of the Carnegie Museum. 



these cavities. The posterior opening of the nares is about the same 



size as in Arctoviys, but it opens more downward, as the basisphenoid 



is more steeply inclined. 



Measurements. 



Mm. 



Length of skull to posterior portion of zygomatic arch 63 



Width of skull between orbits 30 



Greatest width of skull including zygomatic arches 60 



Depth of skull at anterior beginning of zygomatic arches 22 



Width of palate between first molars 15 



Length of molar-premolar series 16 



Antero-posterior diameter of upper incisor 7 



Transverse diameter of upper incisor 5 



Mylagaulus Cope. 



Several lower jaws and teeth of Mylagaulidae from the Loup Fork 

 of Montana throw considerable light on the tooth structure and the 

 development of the characteristic dentition of this family. No skulls 

 were found, but the upper dentition undoubtedly kept pace approxi- 

 mately with the lower. This is true of the species Cerafogauliis 

 rhinoceros Matthew, and Mylagaulus IcEvis Matthew in the American 

 Museum of Natural History. 



The specimens here described may not all belong to the genus 

 Mylagaulus, but they undoubtedly belong to the same family. I 

 think it better to include all under Mylagaulus until more complete 

 material is found. 



In one specimen {M. prisfinus) the erupting large premolar was 

 apparently pushing out with its posterior portion a short-crowned, 

 long-rooted tooth. Its anterior portion is replacing a tooth only a 

 portion of one root of which remains. In two other specimens the 

 large premolar has missed this short-crowned, rooted tooth ; or the 

 anterior portion of the latter has apparently been absorbed and its 

 posterior portion still remains between the large premolar and the 

 first prismatic molar. 



There can be little doubt that this last rooted tooth is a milk molar. 

 It is not at all prismatic, has long roots, is much worn in the young^ 

 animal, and in one case is being shed. The permanent premolar and 

 the two permaneiit molars are prismatic. In the descriptions which 

 follow, the rooted tooth above described will be designated as the 

 fourth temporary molar, dm^^. If the above conclusions be true the 

 large, permanent, prismatic premolar replaces two temporary molars. 



