162 University of California Publications in Geology [Vol. 7 



cisive foramina are wide, and open, and they are two very dis- 

 tinct parallel ridges running from these foramina to the base of 

 the premolars. The skulls of Thomomys I. navus show incisive 

 foramina differing in that some are more open and that the ridges 

 are sometimes present and sometimes lacking, due probably to 

 the age of the individual. In no. 5622 the incisors apparently 

 pass abnormally far back of the anterior roots of the zygomata, 

 but the specimen is broken so that the incisors are exposed, and 

 it is possible that the length appears abnormal compared with 

 unbroken skulls. In this specimen the incisive foramina are 

 not as wide open, nor are the ridges as apparent. One lower 

 mandible has very small teeth, and yet it is evidently not that 

 of a young individual, and the angle of the jaw is much ex- 

 tended. If these various characters were all combined in one 

 individual there would seem to be ground for separation of a 

 subspecies, but in view of the fact that each one presents only a 

 slight difference, and that the material is so fragmentary, they 

 have been placed under the living species. They seem to come 

 under the T. leucodon group rather than under T. monticola, 

 because in T. monticola the root of the incisor curves down ab- 

 ruptly in front of the anterior root of the zygomata, while in 

 T. leucodon the root of the incisor slopes off more gradually and 

 passes almost directly back of the root of the zygomata. 



EEETHIZON EPIXANTHUM Brandt 

 The genus Erethizon is known only from Samwel Cave, but 

 it is the most fully represented of all the rodents. A nearly 

 complete skeleton with the skull, no. 11376, was found, and also 

 a skull with the left ramus of the lower jaw, no. 8901. 



A comparison of the fossil specimens with a Recent individual 

 from the collection of the California Museum of Vertebrate 

 Zoology, no. 16216, from the Whitney Creek, Tulare County, 

 California, and with five skulls from the collection of the U. S. 

 National Museum, nos. 108991, 109142, and 109143 from Tuol- 

 umne Meadows, and nos. 109276 and 11082 from Mt. Dana, 

 California, showed no points of difference either in the skeleton 

 or skull which would separate the fossil form from the living 

 species. Specimen no. 11276 is a comparatively large skull, and 



