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Show • 242 EXPLOI~ATION OF THE OAN'ONS OF THE COLORADO. only three pairs of teats-two pairs inguinal, and one pair, longo intervallo, pectoral. These are very cou picuou , on naked scabrous spaces, and the th i 11 coarse fur would hardly, I think, hide others if they were present. In som0 species of this family I have distinctly recognized six pairs. I observe n') sexual peculiarities in size or color. The geographical distribution of the species has been already indicated as far as my present materials go. I am in possession of no information respecting its habits, which, however, may be presumed to be the same as those of its congeners; though the weaker feet and proportionally smaller pouches may indicate that the fossorial character is not pushed to such an extreme as is the case with G. bursarius. The written history of the species is brief and precise; the nnme having hcen only introduced in 1852. Audubon and Bachman's account is from Lc Conte, and Baird redescribes LeConte's type. It is quite possible, and, indeed, probable, that this second Mexican species has figured at times under the name of mexicanus, but it would only tend to obscure a matter now clear to drag any such point to light. Now that we know of two perfectly good species in Mexico, the less said about the Tucan of Hernandez, or any similar subsequent uncertainties, the better. A Saccopho1·us quachil, from Guatemala, was named by Dr. Gray in 1843, tho\.1gh I cannot find that the species was ever clescribcd. But through the kind offices of Mr. R. B. Sharpe, who, with the assistance of Mr. Gerrard, examined the type still in the British Museum, at my request, I urn informed that it is the animal first described, though subsequently named, by Dr. LeConte. The foregoing pages include all the species of Geomys with which I am acquainted, and account probably for all the names which have been introduced excepting one, G. heterodus, recently described, from Central America, by Professor Peters, of which I know nothing. (" Uber neue Arten dcr Saugthier-Gattungcn Geomys, Haplodon und Dasypus." < Monatsberichte Acad. Wissensch. Berlin, 1H64, Mar. 17, pp. 177-180.) COUES ON GEOMYS AND TBOMOMYS. Genus 'THOMOMYS, Maxim. Oryotomya, pt. EYD. & GEnv., Mag. Zool. vi, 1836, 23. Thomomya, MAXIM., N. Act. Aoad. Crus. Loop. xix, 1839, 383. 243 (In addition to the foregoing, all the synonyms of Oeomus, q. v., ho.vc been applied to this genus.) The readiness with which the species of Geomy:s may be recognized and defined, is a measure of the difficulties encountered in the genus Thomomys, where, with the exception of T clusius, the several forms into which the genus has become differentiated are not yet sufficiently stable to permit of positive, precise determination. An:cr bringing to bear upon the subject an unusually protracted stucly, in the course of which I have critically examiuccl a hund reel or more specimens, I am forced to the conclusion that not a single one of the six or eight currently recognized species is susceptible of satisfactory diaguosis. No descriptive formula can be devised to mark off the characters of any one set of specimens, so completely is the whole series linked together. Nevertheless, it is easy to recognize three extreme of variation (i. e., of differentiation), selected specimens of which would not be confounded by the most careless observer; and it would be as unscientific to ignore these various phases of the genus, as to force them unnaturally apart in an attempt to iguorc the still extant links by which they are bound together. There is an unmistakable average of characters, which serves for the recognition of three climatic or geographical races, conspecies or subspecies, which may be de cribed in terms perhaps covering 7f> per cent. of existing individuals; but the remainder cannot be th us dispo ed of. In other words, the causes which have been operative in modifyiug un original Thorrwmys stock have been only incompletely effectual in the formation of species. We clearly observe the tendency of those modifying influences to which the genus has been subjected; but we note with equal clearness the incompleteness, up to the present time, of the result. Nor is this by any means an exceptional case; on the contrary, positive diagnosis of forms, or specific clistinctions in the proper sense, become impossiule, 1:n perhaps a majority of cases, when sufficient series of specimens arc examined. As I ./ have frequently remarked before under cliffcrcnt modes of expression, the |