446 U. Ss. P. R. R. EXP, AND SURVEYS—ZOOLOGY—GENERAL REPORT. 
good characters for the establishing of groups, younger specimens having more folds than old 
ones of the same species, for, as the accessory tubercles along the edges of the main ones are 
ground off, the two folds originally between these are lost. In the opinion of Burmeister, 
therefore, the shape, number, or character of these folds cannot be relied upon for defining 
groups, or even of genera, for the reasons above mentioned. 
The roots of the molars are much in proportion to each other as are the tubercles, although 
fusion of the pairs of roots is more frequent and intimate than that of the tubercles. The first 
upper molar has one large root in front, and behind this on the outside are two small roots, on 
the inside one larger. The second upper molar has two roots externally, one large one inside. 
The third has generally the same number as the second, but they are smaller. The roots of 
the lower molars are somewhat differently arranged ; the first or anterior tooth has four roots, 
but of these, one anterior and one posterior are large roots, and between these, two other small 
ones opposite each other. The second and third molars have each two roots, the posterior some- 
what the larger. 
The genus Neotoma, and to some extent Sigmodon, are somewhat different from what has been 
described above as more particularly characteristic of the Hesperomys group. Their peculiarities 
will, however, be adverted to more fully under their respective heads. Thus, the first upper 
molar of Neotoma (N. occidentalis, 1662) has but three roots, two external, and one internal and 
between the other two, which are also anterior and posterior—all of nearly equal size. The 
second molar has two roots—one anterior, one posterior—the anterior, however, looking as if 
formed by the fusion of three small contiguous ones. The third has likewise two roots, the 
anterior composed probably of two. 
I have never seen, either in Sigmodon or Neotoma, any tubercles on the molars, even in the 
youngest teeth examined, the crowns being plane with loops of enamel. Some of the other 
generalizations do not apply very well to our North American species, 
In further illustration of the peculiarities of the American muroids, it may be remarked that 
the upper lip is generally thicker, more fleshy, and broader across the nose than in the Old 
World species. 
The American Sigmodontes, as above stated, belong to the genera Hesperomys, Reithrodon, 
Holochilus, Sigmodon, and Neotoma. All these occur in North America, excepting Holochilus, 
These may be most readily characterized as follows: 
Hesperomys. Small, mouse-like. Molars narrow, with two longitudinal series of alternating 
points. 
Reithrodon. Like the preceding, except that the upper incisors are grooved anteriorly. 
Holochilus. Rat-like ; upper lip not fully cleft ; last upper molar equal to the first. 
Sigmodon. Arvicoloid ; upper lip scarcely cleft; two posterior lower molars with the enamel 
loop sigmoid ; lobes all rounded. 
Neotoma. Rat-like; teeth arvicoloid; crowns composed of triangles ; posterior lower molar 
with the enamel outline somewhat 8-shaped. 
