146 



University of California Publications. 



[Geology 



the bone-bearing deposits have proved destructive to all bnt 

 the larger forms. On the contrary, the caves, having acted to 

 a greater or less extent as receptacles for the accumulation 

 of surface material, might be expected to afford a more complete 

 faunal record, as the conditions governing preservation are 

 usually quite favorable. This expectation has been fully 

 realized. 



A large number of the species of Quaternary mammals col- 

 lected from the caves are new. The greater number of the new 

 species were obtained from the Potter Creek Cave, the explora- 

 tion of which has already been described.* New material was 

 also secured from the well known Mercer's Cave situated near 

 the town of Murphys in Calaveras County. In the description 

 of the following species the writer is particularly indebted to 

 Dr. C. H. Merriam for information concerning the relationships 

 of the rodents. 



THOMOMYS MICRODON, n. sp. 

 PI. 19, Figs. 1-3. 



Type. — No. 5738, Univ. of Cal. Palae. Coll. The anterior portion of a 



skull without the mandible. 

 Locality. — Potter Creek Cave, Shasta Co., California. 



This species closely resembles Thomomys mazama Merriam 

 C. H.. from which it differs in having a very prominent ridge 

 on the side of the rostrum, marking externally the position of 

 the alveolus of the superior incisor. The fossa above the ridge 

 is deep. Thomomys niger Merriam C. H. has the fossa behind 

 the incisor ridge almost as deep as in the fossil form, but differs 

 in having the rostrum heavier and the molars much larger. In 

 the fossil form, the rostrum is rather shorter and broader than 

 in Thomomys mazama, and the premaxillae are less pointed. 



The new species belongs to the yellow toothed division of the 

 genus. The incisors are flattened on the anterior side, the 

 anterior outer corner is angular and the teeth are sharply 

 decurved. 



*Science, N. S., Vol. XVII, No. 435, pp. 708-712, May 1, 1903; Univ. 

 of Cal. Publ., Am. Arch, and Eth., Vol. 2, No. 1, 1904. 



