160 University of California Publications in Geology [Vol. 7 



if taken in conjunction with the fact that the rostrum and in- 

 cisive foramina really were shorter, would further strengthen the 

 specific differences of T. spelaea; but in the light of the fact that 

 the two latter points have been disproved, the fact that the tooth 

 row is longer would hardly seem a ground for specific differen- 

 tiation. In regard to the enamel loops of the lower molars being 

 more evenly balanced in the fossils, no difference between them 

 and those of N. c. occidentalis could be found and the writer has 

 placed all the specimens from both caves under this living species. 



Some of the lower mandibles from both caves have been 

 referred to another species, N. fuscipes, but there is a well- 

 marked difference in the shape of the mandibles of the two forms, 

 and the fossil specimens can all be readily distinguished as 

 belonging to the N. cinerca group. N. cinerea and N. fuscipes 

 have been taken in the same localities, but as a rule they do not 

 occupy the same territory and it is evident that the fossil speci- 

 mens are those of the former group, which now lives among rocks 

 and might have lived in the caves, rather than those of the latter 

 group, which build houses of sticks above ground. 



MICEOTUS CALIFOEN1CUS (Peale) 



Three specimens only of this genus occur in the caves, two 

 from Samwel Cave and one from Potter Creek Cave. The species 

 M. calif ornicus is one which inhabits comparatively dry ground. 

 It is, therefore, not surprising to find it in the cave deposits, but 

 the fragile nature of the skull would make its preservation un- 

 common under any circumstances. A lower mandible from 

 Samwel Cave has a complete set of teeth, and the one specimen 

 from Potter Creek Cave is a portion of the skull with dentition, 

 so that the species can be definitely placed. 



THOMOMYS MICEODON Sinclair 



This seems to be a well-defined species described by Sinclair 8 

 as differing from Thomomys mazama "in having a very promi- 

 nent ridge on the side of the fossa, marking externally the posi- 

 tion of the alveolus of the superior incisor, and with a deep 

 fossa above the ridge." Also the rostrum is short and broad. 



8 Univ. Calif. Publ. Bull. Dept. Geol., vol. 4, p. 146, 1905. 



