MURIDA—SIGMODONTES—SIGMODON. 3D 
divided into five separate dentine islands (when the abutment is perfect). The 
nicks in the border of the tooth between these folds are more open than in 
any of the other teeth; in fact, approaching the open reéntrances of eotoma 
The two last under molars differ much from the first, and are almost precisely 
like each other; they have usually but one perfect reéntrant loop on both 
inside and outside, and, as these loops alternate, an appearance something like 
the letter S is produced. But the imperfection, and especially the incon- 
stancy of this pattern, has been already mentioned, and is further shown below. 
Often, in case of the middle lower molar, there is another imperfect loop, either 
internal or external—or two such, one external, one internal; these we have 
not noticed on the back tooth, where the “sigma” is best shown. 
The teeth of the Mexican skulls before us, including S. ‘“toltecus”, offer 
nothing noticeably different from the ordinary style. M. De Saussure figures 
(7. c. pl. ix, f. 3") an average example—perhaps rather elderly, however. Our 
No. 7510 is still older, showing many of the reéntrant folds dissevered from 
the surrounding wall, and forming conspicuous islands in the dentine area. 
The teeth of an aged Sigmodon (No. *%7s°, South Carolina) show conelu- 
sively that the progressive changes of the molar crowns are as described 
in Vesperimus, although Sigmodon, like Neotoma, loses its tubercles so early 
that we have not observed the primitive unworn condition. The senile con- 
dition that the molars of this specimen have reached may be said, in a word, 
to be the penultimate one, in which the reéntrant loops of enamel, though 
still evident, are nearly severed from their connection with the general 
envelope—the peninsulas are almost islands in some places, in others have 
become quite isolated. These molars are nearly worn down to the roots. 
The only further change of which they would have been susceptible had the 
animal lived, is the final rubbing out of these islands, when the teeth would 
have presented a single continuous depressed dentine area, irregularly bounded 
by the external sheet of enamel. The front upper molar shows two external 
and two internal in-lying folds; the former still perfectly peninsular, the latter 
almost isolated. The middle shows one perfect internal peninsula and two 
external folds; the anterior one of which is already insular, the posterior 
nearly so. The back upper molar is in the same condition as the middle one. 
The front under molar shows two internal peninsulas alternating with an 
external peninsula and an external island. Both the other under molars show 
one external peninsula; on the middle one the internal loop has become 
insular, while on the last one the same loop remains peninsular. 
