Notes on Thomomys bulbivorus. 115 



No. far. Date. Length. Tail. Hind foot. 



HI 9 May 5, 1890 260 74 37 



HI J 1 May 12, 1890 280 73 41 



ilf c? May 12, 1890 255 67 38 



The longest hind foot among thirteen specimens of T. bottse 

 from Nicasio, Marin county, California, is 32 mm. ; shortest, 

 28 mm. ; average, 29.7 mm. 



Thomomys bulbivorus differs from T. botttv so greatly in color, 

 as well as in size, that a detailed comparison of the two animals 

 is scarcely necessary. In T. bottse the prevailing tint throughout 

 is wood-brown, more or less mixed with russet dorsally and 

 blackening about the mouth, muzzle, and cheek-pouches. The 

 latter are here, as in T. bulbivorus, lined with white ; the area 

 between, however, is usually dusky, sometimes more or less 

 marked with white, but never, or at least very exceptionally, 

 wholly white. 



The skull of Thomomys bulbivorus, in addition to its very much 

 greater size, differs from that of T. botUe in many details of 

 structure. The occipital portion is broader and flatter (ratio of 

 height from inferior lip of foramen magum to mastoid width 50 

 in bulbivorus, 54 in bottse,} and the fronto-palatal depth propor- 

 tionally greater. The dorsal aspect shows no decided points of 

 difference, though in T. bulbivorus all ridges and muscular at- 

 tachments are more strongly accentuated. Ventrally, however, 

 important differences at once present themselves. That surface 

 of the exoccipital which appears on the ventral aspect of the 

 skull immediately laterad of the condyle is in T. bulbivorus oc- 

 cupied by a deep groove running obliquely to the axis of the 

 skull, while in T. bottse the surface is almost -flat. The basi- 

 occipital is much broader in proportion to its length in bulbivorus 

 than in bottfe; the audital bullse of the former are much flatter and 

 less inflated than in the latter. The form of the pterygoids differs 

 markedly in the two species, those of T. bulbieorus being much 

 the larger and strongly concave internally with hamulars con- 

 verging at the tips, while in T. bottte these bones are flat, with 

 hamulars divergent posteriorly. Both foramen magnum and 

 external nares are broader in proportion to their height in 

 T. bulbivorus than in T. bottse. Except in size, the mandibles 

 and teeth of the two species show no distinctive characters. 



The following table of cranial measurements and ratios of 

 Thomomys bulbivorus and T. bottse will serve to illustrate some of 

 these differences in greater detail. 



