OCR Text |
Show 226 EXPLORATION OF THE OANONS OF THE OOLORADO. considerable di tance. I have not been able to discover any more; and as the sume number and position have been found to hold in G. tuza, mexicanus, and hispidus, such is probably the normal case in this genus; though in species of Tlwmomys I have distinctly recognized six pairs. The character of the incisor dentition remains for special notice, as a diagnostic feature of the species among all itR congeners. Two grooves upon each upper incisor always persist distinctly. One of these is a sharp, fine line of impression, running along the inner margin of the tooth, ubout the distance of its own width from the edge. The other is a much larger, more profound, and wider sulcus, which fairly bisects the remaining surface, leaving an equal plane area on either side, exclusive of the small portion cut off by the fine marginal groove. This main groove varies a good deal in depth and width in different specimens, and, moreover, is itself sometimes sulcate; that is to say, this excavation sometimes presents, on the outer side, a fine ridge, which marks off a secondary groove within the first. rrhe same thing occurs in the single-grooved species-castanops, mexicanus, and hispidus. But this supplementary cariuation of the main groove is not always perceptible, and is generally liable to be overlooked, it is so fine. When most strongly marked, it is just like the inner marginal groove itself; and each tooth seems to consist of two similar halves. Coloration.-Throughout this family, the coloration is general and diffused; there are no strong special areas of parti-coloration. Most of the species (if not all) of both Geomys and Thomomys occur under two states of pelage, which we may call the chestnut and the plumbago. The difference is much like that between the gray or cinereous states of young Hesperomys, Neotoma, &c., and the brighter-colored adults of the same. But, in the present case, it does not appear to be a matter of age, since full-sized, if not mature, specimens are plumbago-colored. If the dark style of pelage be not wholly fortuitousi. e., pure melanism-it is, at any rate, as yet unexplained. To these generalities of coloration. it is to be added, that there is a tendency to indefinite albinism ~f the tail, feet, and parts about the mouth. Whatever the phase of coloratwn may be, the character of the pelage is constant. As in all the other l'ipecics, excepting G. ldspidus, the fur is remarkably soft, fine, and lustrous, remin<.ling one of that of the mole; yet not of the short, close, velvety-· UOUES ON GEOMYS AND THOMOMYS-G. BURSARIUS. 227 pilous character seen in that animal, but long and fl uffy. 'rhe coat appears to be shed from before backward by a steadily progressing process, as wituessed by the definite lines of demarcation frequently observed. 'rhe plum beous basal portions of t he hairs, uniform to the very roots all over the body, are of such extent, and the colored terminal parts so short to · correspond, that more or less of this color appears on all but the most daintily prepared specimens, and plumbeous is the prevailing tone of the under parts. The normal coloration is a dull reddish-brown, or impure chestnut, of varying intensity, frequently with a still duller muddy-brown superficial cast difficult to describe. This is the character of the upper parts, where a dusl{y dorsal area may or may not be appreciable. It gives way on the sides to the plumbeous of the under parts, which is overlaid with a hoary-brown or muddygray. The lips, chin, feet, and even legs, ann the tail. are usually more or less white, the extent and purity of this white being wholly indeterminate; it is sometimes wanting; sometimes the tail is variegated with white and brown. There are also liable to be irregular white patches on the belly. The soles and palms, when not soiled by adventitious substances, are nearly colorless. The incisors, as usual, arc orange-faced in the adult state. The claws are of an indefinite pale-hrown color, often variegated with extravasated blood. The plumbago state, in which some specimens as large as any others are found, is entirely different, and does not appear to shade into the normal phase. Here the color is exactly that of a lead-pencil mark on white paper; but such is the gloss of the fur that violet, purplish, or even brassy reflections are shown witb different lights. It is an intensi fication of the ordinary plumbeous basal portion of the hairs, and its extent over the whole fur. In this condition, white paws and tail, and other irregular patches of albinism, also· occur. 'rhe plumbcous is seen in its purity only above; below, the fur is pointed with muddy-brown or gray. HISTORY.-Although its written hLtory docs not date so far back as the early notices of the '' Tucan" (G. mexican us), this species was the first to be introduced to notice under a scientific name, and with a (supposed) scientific description. Dr. George Shaw was the physician who attended at the birth of the pecics, which he called Mus bunarius, giving a recognizable descrip- |