COUES ON GEOMYS AND THOMOMYS. 243 
Genus THOMOMYS, Maxim. 
Oryctomys, pt. EYD. & GERV., Mag. Zool. vi, 1836, 23. 
Thomomys, MAXIM., N. Act. Acacl. CSRS. Leop. xix, 1839, 383. 
(In addition to the foregoing, all the synonyms of Geomys, q. v., have been applied to this genus.) 
The readiness with which the species of Geomys 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. After bringing to bear upon the subject 
an unusually protracted study, in the course of which I have critically exam 
ined a hundred 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 diagnosis. 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 extremes 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 ignore 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 sub 
species, which may be described in terms perhaps covering 75 per cent, 
of existing individuals ; but the remainder cannot be thus disposed of. In 
other words, the causes which have been operative in modifying an original 
Thomomys 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 distinctions in the proper sense, become impossible, in perhaps a 
majority of cases, when sufficient series of specimens are examined. As I 
have frequently remarked before under different modes of expression, the 
