424 



University of California Publications. 



[Geology 



ing of a strongly curved ridge formed by the entoconid, hypo- 

 conulid, and hypoconid. In both teeth the metaconid is much 

 higher than the protoeonid, and in P 4 it is decidedly recurved. 

 There is a fossette between the protoeonid and metaconid. The 

 entoconid and hypoconulid form a quarter-circle, running from 

 the metaconid around to the triangular hypoconid, and enclosing 

 a deep pit containing small tubercles. There is also a ridge 

 between the base of the hypoconid and the protoeonid. All the 

 cones are set obliquely to the median line of the jaw, but those 

 of Mj are more oblique than those of P 4 . The incisor is con- 

 siderably flattened transversely and is small in proportion to the 

 other two teeth. 



A specimen, no. 12544, comprising the right P 4 , half of left 

 P 4 , and a portion of the right ramus of a lower jaw with incisor, 

 from locality no. 1105, on Thousand Creek, is evidently to be 

 referred to A. nevadensis. P 4 of this specimen is similar in 

 pattern to that of A. nevadensis, but larger. The anterior por- 

 tion is much broader, while the heel is comparatively small and 

 rather pointed. The incisor is small in proportion to the pre- 

 molar, although somewhat larger than that of A. nevadensis. 



Measurements. 





No. 12506 



No. 12514 



A.primigenia 



P 4 , median anteroposterior diameter 



5.8 mm. 



7.4 mm. 



6.5 mm. 



Mj, median anteroposterior diameter 



6.3 





4.6 



P 4 , transverse anterior diameter 



5.1 



6.0 



4.3 



M u transverse anterior diameter 



6.4 





5.8 



M,, transverse posterior diameter 



6.9 





5.6 



P 4 , transverse posterior diameter 



5.9 





5.6 



P 4 , anteroposterior diameter of heel 



3.3 





2.4 



Mj, anteroposterior diameter of heel 



4.4 





3.2 



Lower incisor, anteroposterior diameter 



6.4 



6.9 



6.7 



Lower incisor, transverse diameter 



4.1 



4.6 





Length of diastema 



19.1 





13.3 



Base of P 4 to mental foramen 



11.2 





10.7 



Width of ramus over mental foramen 



... 15.2 





11.6 



Comparison with other Species of Arctomys. — Prom the ar- 

 rangement of the tubercles of the teeth it is evident that this 

 specimen must be referred to the genus Arctomys rather than 

 any other genus of the Scinridae. The cheek teeth have three 



