1883. | The Extinct Rodentia of North America. 5I 
chus are represented by numerous iso- 
lated fossettes. The structure of the 
molars in the fossil genus is exactly 
like that which I have described as 
found in Eumys, extending even to the 
details. This is curious, as that genus 
is probably a Myomorph. 
The protrusion of the posterior ex- 
tremity of the alveolor sheath of the in- 
ferior incisor on the outer side of the 
ascending ramus is not exhibited by the 
North American Sciuridæ which I have 
examined, nor by any of the extinct gen- 
era herein described, excepting Castor 
and the Geomyidæ. Itis seen in a lesser 
degree i in the house and wood mice, the ~% 
jumping mouse and meadow mouse, all ig, 52-2, Gymnoptychus 
Muride. Agee Cope, from the White 
d of Colorado. a, 
Two species of this genus are certainly eh size; d-d, enlarged, 
e, lower jaw of Gymnoptychus 
known. They belong to the White $- ebhus: Copey eas DÈ 
River horizon of Colorado. They dif- oad same locality. Orig- 
„fer, so far as known, chiefly in size, and '™*" 
in the proportions of the inferior premolar tooth. See Fig. 5. 
Mensıscomys Cope. 
This genus is readily distinguished from all the others here 
treated of, by the complexity of the structure of its molar teeth, 
and the curious resemblances that some of them present to the 
molars of the hoofed mammalia. They are without enamel inflec- 
tions, and the triturating surface exhibits two external and one 
internal crescentic sections of the investing enamel. On the sec- 
ond superior molar there are three external crescents, and the 
first molar is simply conic. Between the inner and external cres- 
cents there are the curved edges of enamel plates directed ob- 
liquely and transversely. The grinding surfaces of the inferior 
molars display; in the unworn condition, curved transverse crests, 
connected longitudinally on the median line; on wearing, the 
lateral emarginations of the enamel become shallower, disap- 
pearing from the inner side, but remaining on the outer. Incisor 
teeth not grooved. Foramen infraorbitale anterius small inferior, 
