﻿July, 1896.] 



SUBGENUS CHILOTUS. 



61 



Bony paiate, — The palate is normal and calls for no further remark. 



Enamel xjattern in general. — The enamel foldiug (fig. 32) is like that 

 of the tetramerodont species of MicrotuSj except that the back upper 

 tooth is a little simplified. 



Front loicer molar. — The first mandibular molar is exactly like that 

 of tj'pical Microius. 



JBaclc upper molar. — The back maxillary tooth contains a transverse 

 anterior loop, two lateral closed triangles, and a somewhat lengthened 

 terminal loop. The latter has at each side of its base a conspicuous 

 angle, the outer one of which is often isolated as a third closed triangle. 

 The tooth has six salient angles, two to each of the transverse loops 

 and one to each of the closed triangles. 



Other teeth. — As already stated, the remaining teeth are formed 

 exactly as in tetramerodont Microtus. One specimen from British 

 Columbia has the lateral triangles closed in the back lower molar. 



Mamma\ — There are eight mamnne, four j)ectoral and four inguinal. 



Feet. — Soles moderately hairy from heel to tubercles; plantar tuber- 

 cles five, all well developed; claws on hind feet 



longest; front feet not modified like those of typical 

 Fitym ys. 



Fur. — The fur is shorter and more dense than 



in true Microtus, but the modification is not car- yig. 32.— Enamel pattern of 

 ried so far as in Microtus {l*iiy)nys) j^inetorum. moiavteeth. Microtus {GH- 



General remarlcs. — Chilotus combines the mam- ^"f^'^) (^regom. (xs.) 

 mx and foot pads of Arvicola with the nearly tj'pical enamel i^attern of 

 Microtus and has a form of skull peculiarly its own. In general it is 

 modified in the same direction as rHymys, but to a much less degree. 



Great stress has been laid on the form of the ear as a chara(;ter of 

 this subgenus. In the original description^ Baird says: 



A specimeu in alcohol, from Steilacoom^ received siuce the i^receding description 

 Tvas prepared, is, in size, mucli as described. The ears are low, orbicular, the mem- 

 brane thickened, the margins or conehal jiortion much inflected or incurved, like a 

 half-open apple blossom, the concha being inflected all round. The antitragus is 

 "well developed, but rather low. The surfaces of the ear appear perfectly uaked, 

 with, however, a ciliatiou of long liairs toward the roots of the concha, on the dorsal 

 surface. A close examination of the auricle iuthe dried siiecimen shows a few scat- 

 tered, A-ery short, wliite hairs. 



The structure of the ear, though in many respects simihxr to that of A. 2)inetonim, 

 is yet essentially diflerent. Thus the upper and lower roots of the margin of the ear 

 meet anteriorly so as to form even a low rim to the meatus anteriorly, completely 

 inclosing the aperture; the edge of the concha is inflected; the region inside the 

 auricle, around the meatus, naked, and the antitragus so much developed as to be 

 capable of completely closing the meatus. In A. pinetoruiii the roots of the Tipper 

 and lower margins of the ear are widely sei^arated, by a space of a quarter of an 

 inch, the space between these roots and anterior to the meatus perfectly plane ; the 

 edges of the concha, or of the auricle, not inflected at all; the inner space around 

 the meatus partly hairy ; the antitragus very slightly developed, not valvular, nor 

 capable of closing the meatus at all. 



' Mamm. Am., p. 538, 1857. 



