W. S. Sinclair — Mylagavlodon. 143 



Art. XIII. — Mylagaulodon, a New Rodent from the Upper 

 John Bay of Oregon; by W. J. Sinclair. 



Dr. W. D. Matthew, in his revision of the Mylagauli, 

 enumerates* five species referable to two genera, all of which 

 are either from the Loup Fork or the Deep Kiver beds. No 

 representative of this family has so far been described from 

 the John Day. Two years ago, the members of the University 

 of California party discovered the skull of a small rodent in 

 the uppermost beds of the Upper John Day on Johnson 

 Creek, Wheeler Co., Oregon. The specimen is unique, dif- 

 fering from anything known. It is believed to represent a 

 transitional stage between Allomys hippodus of the Middle John 

 Day and the Loup Fork Mylagaulus. The type specimen (see 

 fig.) is a poorly preserved cranium (Pal. Museum, No. 1652). 



Fig. 1. 



The dentition is imperfectly preserved, but the dental formula 

 was probably reduced below that of Allomys. The third 

 superior premolar is a simple cylindrical tooth, as in that genus. 

 The fourth premolar is hypsodont. On the triturating surface 

 of this tooth there are four antero-posteriorly elongated lakes, 

 with a fifth small circular lake situated between and external 

 to the two large outer lakes. On the outer side of the antero- 

 external lake there is a straight enamel ridge narrower than the 

 circular bands inclosing the lakes. This ridge and the small 

 circular lake would probably disappear in the worn tooth. 

 The external side of the crown is flat with a prominent postero- 

 external angulation, which suggested the specific name angu- 

 latus. The antero-external angle of the crown is rounded. 



* Bulletin Am. Mus. Nat. Hist., vol. xvi, pp. 291-310. 



