1912] Kellogg: Pleistocene Rodents of California 



167 



THOMOMYS BOTTAE PALLESCENS Ehoads 



The specimens of Thomomys represented in the Rancho La 

 Brea fauna show quite a range of variation due to differences in 

 age, the younger individuals with a short rostrum and slender 

 jugal and with the rami of the lower jaw small, the older ones 

 with more elongated rostrum and larger rami. They have all 

 been placed under the subspecies T. b. pallescens because they 

 present no greater range of variation than is present in this 

 species. The teeth of two specimens, no. 1212, a left ramus of the 

 lower jaw with M x and M 2 , and no. 12418, a right ramus of 

 the lower jaw with incisor and P 4 , M x , M 2 , are slightly different 

 from those of T. pallescens, but in the case of no. 1212 the effect 

 is partially due to a break in M 2 which gives it a very broad 

 appearance. However, the teeth of the fossil form are slightly 

 heavier than those of typical T. pallescens. 



PERODIPUS AGILIS (Gambel) 



One specimen, a portion of the skull showing the premaxil- 

 laries and frontals and three teeth, P 4 , M 1 , M 2 , embedded in a 

 piece of asphalt, represent this species. The teeth make it refer- 

 able to Perodipus rather than to the externally closely related 

 genus of pocket rats, Dipodomys. 



8YLVILAGUS AUDUBONI (Baird) 



This species is the best represented rodent found in the Rancho 

 La Brea deposits. There are eleven specimens, mostly mandibles. 

 There are two fragments of skulls, one of which has a full set of 

 teeth on the left side. The fossil forms are those of fairly 

 young animals, thus bearing out the theory already stated by 

 Dr. J. C. Merriam that the very young animals through lack 

 of discretion, commonly fell victim to the asphalt. The speci- 

 mens have been referred to 8. auduboni instead of to the sub- 

 species sanctidiegi, because the latter is based mainly on a 

 geographic distribution, and although it is supposed to differ 

 from 8. auduboni in having narrower and more slender jugals 

 and a broader palatal ridge, a comparison of skulls of the two 

 forms did not bear out the latter distinctions. 



