1915.] 



THOMOMYS BULBn'ORUS GROUP. 



41 



Color. — 'Winter pelage: Dark sooty brown, the plumbeous basal 

 color showing through on belly; ears and nose blackish; chin and 

 throat and usually small anal patch white; feet usually streaked or 

 mottled with white. Summer pelage: Washed with rusty brown 

 above and below. Young: Like summer adults but very thinly 

 haired, the skin showing through on belly. 



SlcuU. — Short and wide with zygomatic arches usually widest 

 posteriorly; rostrum, slender; nasals short, falling considerably short 

 of premaxillse at both ends; pterygoids convexly inflated and 

 divided by narrow fossa; bullae rather small and narrow but auditory 

 meatus unusually open or dilated. Dentition rather light; incisors 

 slender, greatly protruding, and usually white tipped; groove on inner 

 edge of upper incisors very obscure. 



Measurements.- — Average of 5 adult males from Salem and Beaver- 

 ton, Oreg. : Total length, 300 ; tail vertebrae, 90 ; hind foot, 42. Largest 

 male: 302, 99, 43. Average of 4 adult females: 271, 81, 39. Slcull 

 ( (? ad.) : ^ Basal length, 52; greatest length over incisors, 57; nasals, 

 19; zygomatic breadth, 36.5; mastoid breadth, 30.5; alveolar length 

 of upper molar series, 10. 



Remar-ks. — This is the most aberrant form of the genus, marking 

 either an ancestral type or more probably a highly speciahzed de- 

 velopment of adaptive characters. Widely separated geographically 

 from the oottde group, with which it shows the closest affin.ities, it is 

 entirely isolated in the humid, rich-soiled valley of the Willamette. 



The object of the projecting incisors becomes evident when these 

 gophers are trapped. During the dry season the soil of the Willam- 

 ette Valley becomes baked and so hard that digging open gopher 

 holes with a small shovel, knife, or sharp stick is difficult, and but 

 for a granular cleavage would be almost impossible. The adobe 

 earth breaks out somewhat as the segments of a pomegranate, but 

 in coarser granules. Evidently the little irregular clay balls mak- 

 ing up the gopher hills are pried out with the projecting incisors, 

 which by long use have become perfect miners' picks. The upper 

 incisors project actually beyond the tip of the gopher's nose and at a 

 convenient angle for prying out the earth balls. The lower incisors 

 project equally far but from farther back. The fur-lined hps close 

 behind the incisors, so that no earth can get into the mouth. The 

 short bristly fur surrounding the base of the incisors protects the 

 Hps, and leaves the '^pick and shovel" teeth free. That the teeth 

 are regularly used for digging is evidenced by the much-worn tips and 

 edges. The white tips are shown by the lens to be due to the wearing 

 away of the yellow enamel surface. The slight inner groove is usually 

 worn entirely away before it reaches the tip of the incisor. It seems 



iNo. 57322, U. S. Nat, Mus., from Salem, Oreg. 



