1883. | The Extinct Rodentia of North America. 49 
of Protomyide, but differ from any of the genera he has included 
in it, . 
This family is thus defined by Pomel 2 “ Infraorbital foramen 
as large as in the Hystricidz, and by the position of the angular 
apophysis of the mandible almost in the general plane of the hori- 
zontal ramus. The jugal bone, at least in those species where we 
have observed it, is very much enlarged at its anterior portion, 
and the orbit is almost superior.” 
These characters apply to Ischyromys, excepting as regards 
malar bone, which is principally unknown in the latter. | 
Another family, the Ischyromyidz, has been proposed by 
E. R. Alston, for the reception of this genus, to which he thinks 
with me? Plesiarctomys (=Pseudotomus) should be referred. 
He thus defines the family :* “ Dentition as in Sciuridz ; skull re- 
sembling Castoridz, but with the infra-orbital opening large, a 
sagittal crest; no post-orbital processes ; palate broad ; basioccipi- - 
tal keeled.” 
Doubtless Ischyromys belongs to an extinct family, but which 
of the above names is available for it I do not yet know. I would 
characterize it as follows: 
-Dentition as in Sciuridæ, infraorbital foramen large, superior ; 
pterygoid fossa large, with well-developed exterior as well as in- 
‘ ferior walls; a sagittal crest. 
The superior position of the infraorbital foramen and the well- 
Pi ay pterygoid lamine are characteristics found in the 
Murid 
Bats one species of this genus is known, the /schyromys pe 
Leidy. The skull is as large as that of a prairie marmot. 
limbs are comparatively small, so that the animal was not sakes 
arboreal in its habits. 
Sciurus Linn. (true squirrels). 
In this genus the molars are § or j, the first superior small 
when present. The grinding surfaces of the crowns when unworn 
present in the superior series a single internal cusp, which is low - 
and anteroposterior. From this there extend to the external 
border of the crown two low transverse ridges, whose exterior 
1 Catalogue Method. et Descr. de Vertebres Foss. de le Bass. de la Loire, 1853, p- 
w : 
* Annual Report U. S. Geol. Survey Terrs., 1873 (1874), P. 477- 
3 Proceed. Zool. Society London, E p 78. 
VOL. XVIIL.—NO, 1, 
