456 U. S. P. R. R. EXP. AND SURVEYS—ZOOLOGY-GENERAL REPORT. 
external characters. Two of these differ only in the presence or absence of hair on the soles 
and may, perhaps, he best considered as one; the third is similar in skull, hut differs in external 
characters ; the fourth is so distinct as perhaps to he worthy of generic separation. 
The molar teeth appear to he very similar in all, decreasing regularly from anterior to poste¬ 
rior, each about two-thirds the length of its predecessor in the upper jaw, the disproportion 
rather less in the lower. The enamel folds are much more superficial than in the South American 
species, so much so as to render it difficult to detect any particular pattern of indentation. 
When the crowns wear plane, instead of loops we see usually only a slightly irregular border 
of enamel encircling the dentine as a single island. The posterior upper molar is nearly circular, 
and very small; it has no appreciable indentations whatever ; the middle molar has one median 
internal, and two rather obscure external, and one at or near each end ; the third has two 
alternating indentations on each side. In the South American specimen referred to, the inden¬ 
tations, even in the much worn teeth, are as deep and as distinct as in Arvicola or Neotoma; those 
of opposite sides of the same tooth are opposite each other, and almost touch ; there are two on 
each side the anterior, and one median on each side of the middle and posterior, thus presenting 
a very different appearance. How constant this may he I cannot say, hut Waterhouse’s figures 
of skulls represent the same thing in nearly all the South American species. These differences 
in the teeth may he tabulated as follows : 
North American. 
South American. 
Indentations of enamel of opposite sides 
of the upper molars_ 
Alternating_ 
Opposite_ 
Outside of middle molar, with._..._ 
Two indentations_ 
One_ 
Posterior molar___ 
Small; circular, with scarcely a trace of 
indentation. 
Larger, subquadrate, with a deep in¬ 
dentation on either side. 
It is very possible, however, that some South American species may he more like ours in the 
character of the skull, especially the species of Habrothrix. 
The first group embraces species most similar in form to the South American, especially to 
Calomys. Here the size is very small, none exceeding four inches in the flesh to the root of the 
tail. The ears are large, with short close hair ; the tail is two-thirds the length of the body or 
else a little longer. The belly and feet are generally white ; the bright fulvous above of many 
South American mice is only faintly represented in one or two species, their golden yellow under 
parts, never. The soles in one division are hairy to the tubercles, which are usually six in 
number ; in the other division the soles are entirely naked. The tubercles are all large and 
conical; the posterior one especially. The claws are all short and weak ; the hinder longest. 
The hind feet are longer in proportion than the fore ; the hand half the length of the hind foot. 
The bristly hairs of the hack are soft, and only a little longer than the under fur, so that they 
are hut little conspicuous. 
The skull has the superior edge of the orbit rather sharply angular, hut not provided with a 
raised crest or head, such as is described in the South American species. In the smaller ot 
these, however, this crest is said to he wanting, as it is in a skull before me from Chile, in which 
the orbital edge is only sharper than in our species. The molar teeth already described are much 
the same in all the North American groups, and hear a close relationship (judging from descrip- 
