MURIDJ3— SIGMODONTES— SIGMODON. 35 



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 reihitrances of Neotoma 

 The two last under molars differ much from the first, and are almost precisely 

 like each other; they have usually but one perfect reentrant 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 

 (I. c. pi. ix, f. 3 a ) an average example — perhaps rather elderly, however. Our 

 No. 7510 is still older, showing many of the reentrant folds dissevered from 

 the surrounding wall, and forming conspicuous islands in the dentine area. 



The teeth of an aged Siginodon (No. Wsf, South Carolina) show conclu- 

 sively that the progressive changes of the molar crowns are as described 

 in Vetperimus, 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 reentrant 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. 



