364 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Vol.Yl. 



GYMXOPTYCHUS Cope. 



Paleontological BnUetin, No. 16, p. 5 (August 20, 1873) ; Ann. Report U. S. Geolog. 

 Surv. Terrs. 1873 (1874), p. 476. * 



Dental formula: I. |; C. ^; M. f. Crowns of the superior molars sup- 

 porting two crescents on the inner side, and two cusps on the external 

 side opposite to them. Each cusp sends a transverse crest to the con- 

 cavity of the corresponding crescent. The adjacent horns of the crescents 

 are united, 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 borders. Incisors simple. The walls of the 

 alveolus of the inferior incisor produced into a tuberosity on the external 

 side of the base of the ascending ramus. 



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

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

 confirmation of this opinion, I add that the alveolar sheath of the infe- 

 rior incisor is in the vertical plane of the ramus; the incisive foramen 

 does not invade the maxillary bones, and the foramen infraorhitale ex- 

 terius 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 differs in the structure of 

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

 complex than in any of them, excepting Pteromys. 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 

 Fteromys (F. Cuv.) the transverse valleys of the inferior series of Gym- 

 noptycJius are represented by numerous isolated fossettes. The structure 

 of the molars in the fossil genus is exactly like that which I have de- 

 scribed above as found in Mujnys, extending 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 Korth American Sciuridw, 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, Hes- 

 jperomys leitcopus, Meriones hudsonius, and Arvieola riparia, all Muridce. 



Whether this genus possesses a postfroutal process I have been unable 

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

 Sciuridw, 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, femur, and tibia. Most of these are appropriate in size to the 

 G. minutus, which is also the most abundant species. A fragment of a 

 larger femur belongs perhaps to the G. trilophus. 



