144 Jr. J. Sinclair — Mylagaulodon. 



There is no trace of cement externally on the crown of either 

 premolar. Molars are not preserved. The poor state of pre- 

 servation of the cranium prevents a detailed discussion. The 

 infra-orbital foramen is larger than in Allomys hippodus, and 

 is situated close to the zygoma. 



Measurements. 



Approximate length of the skull 46 mm * 



Length of diastema anterior to pm. 3 11 " 



Antero-posterior diameter of enlarged premolar 



crown _ 5-5 " 



Transverse diameter of enlarged premolar crown 5 " 



Antero-posterior diameter of pm. 3 1*5 " 



Transverse diameter of pm. 3 1-5" 



Greatest width of incisor 2*5 " 



The species is referred to a new genus, Mylagaulodon, so 

 named from the resemblance of the enlarged premolar to the 

 teeth of the Mylagauli. In the retention of the conical pre- 

 molar, the genus resembles Allomys, from which it differs in 

 the loss of the antero-external buttress between the outer cres- 

 cents in the fourth premolar, the rounding of the antero- 

 external angulation, and the hypsodont character of the tooth. 

 A somewhat similar arrangement of the enamel lakes might be 

 attained in worn teeth of Allomys hippodus. Mylagaulodon 

 is sufficiently contrasted with the beavers in the antero-posterior 

 elongation of the enamel lakes, in which it agrees with Myla- 

 gaulus, differing from that genus, however, not only in tlfe 

 shape of the fourth premolar, but in the retention of the cylin- 

 drical third premolar as in most of the Sciuridse. It is be- 

 lieved that reduction either in size or number of the molars 

 must have taken place in Mylagaulodon, owing to the lack of 

 space on the maxillary to accommodate three more teeth of the 

 same size as the last premolar. 



The presence of this transitional form in the uppermost John 

 Day is important in the light of the known occurrence of 

 Mylagaulus in the Mascall beds at Cottonwood, Oregon. It 

 is in the Upper John Day that the ancestors of the oldest 

 Loup Fork fauna should be sought with the most confidence. 

 It is possible that the evolution of the Mylagauli took place 

 in the Oregon. 



Paleontological Laboratory, 

 University of California, Berkeley, Cal. 



