RODENTIA-ARVICOLINAE-ARYICOLA OREGONI. 
537 
ARVICOLA (CHILOTUS) OREGON!, Bachman. 
Arvicola oregoni, Bachman, J. A. N. Sc. Phila. Yin, i, 1839, 60. — Ib. in Townsend’s Narrative, 1839,315. 
Acd. & Bach. N. Am. Quad. Ill, 1853, 232 ; pi. cxlvii, f. 3. 
Sp. Ch. —About the size and shape of Arvicola pinetorum, (3j inches). Skull . 92. Fur short, (. 3 of an inch). Head 
short, broad. Ears moderate, barely concealed, quite naked, with a few scattered inconspicuous white hairs. Antitragus 
small. Tail vertebras not one-third the head and body, longer than the head, one and one-half times the hind feet. Soles 
hairy for posteriort hird, (. 65 long). For additional characters see Chilolus, page 516. 
Above, dark brown, without any rufous tint. Hairs with obscure tips of yellowish brown. Beneath, lustrous hoary 
plumbeous ash. Tail corresponding in color to the body, but not sharply bicolored. Feet grayish brown. 
This diminutive Arvicola is about the size of northern specimens of Arvicola pinetorum. 
The head is short and broad ; the whiskers about as long as the head, mixed, gray and. black. 
The ears are moderate; barely covered by the fur ; the upper margin indexed ; the auditory 
aperture is small; the antitragus short and low. The membrane of the ear is rather thick ; at 
first sight it appears perfectly naked on both sides, but a close examination reveals a few short 
scattered hairs, except on the upper part of the convexity, where they are long, as on the neck. 
The feet are moderate, the hinder ones proportionally longer than in A. pinetorum. The sole 
is hairy for rather more than its posterior third. The tail is longer than the head and about 
one and a half times the length of the hind feet; it tapers gently to the tip. 
The fur is compact, close, and has considerable lustre ; it measures about three-tenths of an 
inch, rather less beneath. The prevailing color above is a dark brown, without any distinct 
rufous or chestnut; the hairs are tipped with yellowish brown, but so much overlaid with black 
tipped hairs as to produce a fine and equal mixture of the two. The under parts are ashy 
plumbeous. The feet are light grayish brown. The tail is brown above, plumbeous ash 
beneath, the colors passing insensibly into each other. 
Skull. —The skull is short, broad, and depressed ; less so, however, than in A. pinetorum. 
The interparietal portion is widest at its posterior portion, the lateral extremities being obliquely 
truncate. The nasal bones and nasal branches of the intermaxillary are of equal length, and 
reach back as far as the lachrymal. The distance from the anterior extremity of the upper 
molars to the posterior base of the incisors is not quite one-third that from the incisors to the 
occiput. The line of upper molars is contained about four-and-a-half times in the total length 
of the head. The width of the head is about 58 hundredths the length. 
The middle upper molar has four triangles, the posterior one exterior. The posterior molar 
has one anterior triangle, one internal and two external, besides the posterior loop, which is 
somewhat Y-shaped, if the posterior outer triangle be included in it; the loop has a re-entrant 
angle indented in its inner side. There are three salient angles on the exterior of the tooth, 
with a long concave line behind them—three inside, or even four, counting the angle behind the 
indentation of the posterior loop. The first and third upper molars are about equal, the middle 
one a little smaller. 
The anterior upper molar on the left side has one posterior triangle, and three on each side, 
besides an anterior trefoil loop, the external lobe of which is obliterated. That of the right 
side is similar, except that there are but two external triangles, the first anterior outer being 
confluent with the anterior loop, and forming the outer lobe of the trefoil wanting in the tooth 
of the opposite side. There are five internal salient angles and four external, besides the anterior 
loop. 
68 L 
