Messrs. Say arid Old on Sigmodon hispiduni. 297 



Superior Jaw. Incisor slightly rounded on its anterior face, 

 truncated at tip : first molar equal in width to the second, com- 

 posed of four very profound, alternate folds, two on each side, 

 extending at least to the middle of the tooth : second molar 

 quadrate, somewhat wider, and a little shorter than the preceding, 

 with three profound folds, extending at least to the middlie, two 

 of which are on the exterior side : posterior molar a little nar- 

 rower, but not shorter than the preceding, with three profound 

 folds, two of which are on the exterior side, extending at least to 

 the middle, the inner fold opposite to the anterior exterior fold, 

 and not extending to the middle. 



Inferior jaw. Incisor obliquely truncate at tip, the acute angle 

 being on the inner side, it originates in the ascending branch of 

 the maxillary bone, passing beneath the molares : molares sub- 

 equal in breadth, inclining slightly forwards; Jirst molar a little 

 narrower than the second, with five profound, alternate folds, 

 three of which are on the inner side : second molar subquadrate, 

 with two alternate, profound folds, the inner one anterior; third 

 molar about equal in length and breadth to the anterior one, but 

 rather larger, and somewhat narrower than the second, with which 

 it corresponds in the disposition of its folds, excepting that they 

 are less compressed. 



■ Tail hairy : feet simple : forefeet four toed, with the rudiment 

 of a fifth toe having a nail : hind feet five toed. 



Observations. The enamel of the molares is thick, but on the 

 anterior face of each fold, excepting the first, it is obsolete. 

 From the arrangement of the folds as above described, it is ob- 

 vious that the configuration of the triturating surface, (occasioned 

 by the folds of enamel dipping deeply into the body of the tooth, 

 in the second and third molar of the lower jaw.) accurately re- 

 presents the letter S, which is reversed on the right side; thus 

 bearing considerable resemblance to the posterior tooth of the 

 genus Spalax, to which also it has a slight affinity in the truncature 

 of tlie inferior incisores. 



The configuration of the intermediate molar of the upper jaw, 

 may be compared to the form of the Greek letter 2, whence our 

 generic name. 



