COCJES ON GEOMYS AND THOMOMYS G. TUZA. 231 
dry and alcoholic). This species, apparently distinct, nevertheless resembles 
G. bursarius so closely, that no general points of difference in size, shape, or 
color can be adduced for their separation. There are, however, certain tan 
gible characters, not necessarily the same as those given by authors. For 
instance, Audubon and Bachman dwell upon certain supposed differences in 
the structure of the pouch ; but their observations rest upon bad taxidermy, 
and have no foundation in nature, the pouches in the. two species being iden 
tical. These authors and some others speak of the upper incisors as single- 
grooved. This, if so, would be a strong feature : but it, unfortunately, is not 
the case ; for, in all the numerous specimens I have examined, the upper 
incisors are double-grooved, as in G. bursarius, the fine second groove being 
perceptible as a delicate line of impression running along the inner margin of 
the tooth. It is perfectly distint, as a rule ; and in no case have I failed to 
recognize clearly at least a trace of it, though in some instances it is faint, and 
liable to be overlooked" if not closely examined. Baird says that this groove 
is obsolete in old age, implying that such is the rule : but, while not doubting 
that such may occur, I must consider it as the exception ; for, as just said, I 
have never yet failed to recognize at least a trace of it. G. tuza, therefore, 
has double-grooved incisors, like G. bursarius ; the point of dental discrep 
ancy lies elsewhere. In bursarius, the main groove bisects what is left of the 
face of the tooth, after subtracting the portion cut off by the inner groove ; 
and this latter is always distinct. In G. tuza, the main groove divides what 
is left of the face of the tooth, after subtracting the portion cut off by the 
inner groove, into two unequal portions, whereof the exterior is the smaller ; 
and the inner groove, always slight, may be faint, obscure, or perhaps some 
times obsolete. This is the whole case, as far as the incisors are concerned. 
The only other character of G. tuza I can appreciate is the nakedness 
of the tail and feet especially the former. It is true that in G. bursarius 
the nakedness of these parts is sometimes noticeable ; but it seems to be not 
carried, except perhaps in extreme cases, to the extent witnessed, as a rule, in 
G. tuza. The latter thus corresponds with G. kispidus in this respect, 
though very different in other features. In the best-marked cases, the tail is, 
perfectly naked beyond the enlarged hairy base ; the skin may be stuffed out 
to the caliber of a stout goose-quill, and has then a peculiar bladdery appear- 
