34 MONOGRAPHS OF NORTH AMERICAN KODENTIA. 
follows: —The upper molars have three roots each: a large interior fang, and 
two smaller exterior fangs, placed side by side; but the anterior of these, in 
the first molar, is as large as the interior, and stands directly to the front, 
instead of to the outside, while this same tooth also often shows a fourth 
root—a small one interposed between the two exterior ones. But the under 
molar roots show us something that may be peculiar—at least, we have not 
observed it in other genera; this is, that they have more than two roots apiece, 
and the disproportion in size of the roots is very great. The front lower 
molar has four fangs: a moderate anterior fang; an enormous posterior fang ; 
and two little intermediate fangs, placed side by side. The next two under 
molars have each three roots: two small anterior ones placed side by side, and 
one enormous posterior fang. The larger fangs seem to result from the con- 
fluence of two small ones; and, in both upper and under teeth, there are some- 
times minute accessory fangs that do not effect a distinct perforation of the 
alveolus, and are, therefore, not enumerated. We think it very likely that 
this mode of rooting may be really characteristic of Sigmodon, but have not 
ventured to include it in our diagnosis, in our uncertainty as to its permanence, 
and our ignorance of what, if any, other sections may show the same thing. 
The front upper molar is larger than the other two; but these are of about the 
same size, since, contrary to a general rule, the back molar does not diminish 
in bulk. All three have externally two deep reéntrant lobes, or folds, of 
enamel, reaching almost across the face of the teeth; the front upper molar 
has two similar interior folds; the other two but one such interior loop apiece. 
In the front molar, the first exterior loop abuts against the second interior 
loop, or else is received in the space between the first and second interior 
loops; similarly, on the other teeth, the first exterior loop abuts against the 
only interior loop, while, in all three teeth, the second exterior loop abuts 
against the postero-internal angle of the general enamel-wall. The abutment 
of these folds is more or less perfect with different specimens, but, generally, 
it divides the enamel-area into several dentine islands. In the under jaw, 
likewise, the molars do not decrease in length from before backward, the 
back molar being as long as the first, and, if anything, even broader; still, 
the complexity of the crown is much greater on the first than on either of the 
others. The first lower molar shows three interior and two exterior reéntrant 
folds of enamel, alternating with each other; the apex of each fold is received 
against the apex of the next succeeding, so that the face of the tooth is 
