227 



GENUS SIGMODON.— Say and Ord. 



DENTAL FORMULA. 



' " = 16. 



Incisive -^ ; Canine g^ ; Molar g^ = 



As the present genus was instituted after a careful examination of the 

 teeth of Sigmodon hi'spidum, by Messrs. Say and Ord, who first described 

 that species ; we think it due to those distinguished naturalists, to give 

 the dental formula in their own words, more especially as this species 

 was named by us, in our illustrations, Arvicola hispidus, we having had 

 some doubts whether it was sufficiently distinct from the arvicolae in its 

 generic characters, to warrant us in adopting the genus Sigmodon, to 

 w^hich we now cheerfully transfer it. 



" Superior Jaw. — Incisor, slightly rounded on its anterior face, truncated 

 at tip ; first molar, equal to the second, composed 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 middle, 

 two of w^hich are on the exterior side ; posterior molar, a little narrower, 

 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 molars ; molars, subequal in breadth, inclining 

 slightly forwards ; first 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 an- 

 terior ; 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." 



OBSERVATIONS. 



" The enamel of the molars is thick, but on the anterior face of each fold 

 excepting the first is obsolete. From the arrangement of the folds, as 



