820 THE WHITE EIVEB FAUNA. 



and the connecting portion sends a transverse crest into the interval between 

 the cusps. The opposed horns of the two crescents each send a crest round 

 the anterior and posterior sides of the crown, of which they form the bor- 

 ders. Incisors simple. The walls of the alveolus of the inferior incisor 

 produced into a tuberosity on the external side of the base of the ascend- 

 ing ramus. 



The above characters define a genus which, when fully known, will in 

 all probability- be referred to near the existing genus Sciurus. In confirma- 

 tion of this opinion, I add that the alveolar sheath of the interior incisor is 

 in tlie vertical plane of the ramus; the incisive foramen does not invade 

 the maxillary bones, and the foramen infraorbUale exterius is a small fissure 

 situated in the inferior portion of the maxillary bone, well in advance of 

 both the orbit and first molar tooth. 



As compared with the existing genera, it difi'ers in the structure of the 

 molar teeth. The arrangement of the tubercles and crests is more complex 

 than in any of them, excepting Pferomys. Thus in all of them there is but 

 one internal crescent of the superior molars, and but two or three cross- 

 crests ; while in the inferior molars the arrangement is unlike that of the 

 superior teeth, the cross-crests being marginal only. In Pteromys (F. Cuv.) 

 the transverse valleys of the inferior series of Gymnoptychus are represented 

 by numerous isolated fossettes. The structure of the molars in the fossil 

 genus is exactly like that which I have described as found in Eumys, ex- 

 tending even to the details. This is curious, as that genus is a Myomorph. 



The protrusion of the posterior extremity of the alveolar sheath of the 

 inferior incisor on the outer side of the ascending ramus is not exhibited by 

 the North American Sciuridcc, which I have examined, nor by any of the 

 extinct genera herein described, excepting Castor and the Geomyidce. It is 

 seen in a lesser degree in Mus musculus, Hesperomys leucopus, Meriones Imd- 

 sonius, and Arvicola riparia — all Muridce. 



Whether this genus possesses a postfrontal process I have been unable 

 to ascertain. Its absence would not in my opinion isolate it from the 

 Sciuridce, as I accord with Dr. Coues in his estimate of the value to be 

 attached to this character. 



Of other portions of the skeleton I possess incomplete humerus, ischium. 



