DAKOTA AND NEBRASKA. 335 



SCIURIDjE. 



This family is represented in the miocene fauna of the Mauvaises Torres by a pe- 

 culiar genus, to which the following name has been given. 



ISCHYROMYS. 



ISCHTROMTS TYPUS. 



Another rodent animal, distinguished by the above name, and belonging to the 

 family of the Squirrels and Marmots, is indicated by the greater portion of a skull and 

 two fragments of lower jaws, discovered by Dr. Hayden at the head of Bear Creek, 

 in association with the remains of Palceolagus Haydeni. 



The skull, represented in figures 1, 2, plate XXVI, was about the size of that of a 

 Muskrat, Fiber zihetliicus, and also approached it in form, though differing from it and 

 the skulls of all recent rodents in many important points. Its form approaches more 

 that of the Beaver than that of the Muskrat, but bears a nearer resemblance to that 

 of the extinct Sleneofiher viciacoisls, as represented in plate 48 of Gervais' Paleonto- 

 logie Francaise, than with any other form of rodent skull with which the writer is 

 familiar. 



The occipital region, much mutilated in the fossil, was vertical as in the Beaver. 

 In the upper view, the cranium appears proportionately neither so broad nor capacious 

 as in the latter animal, and its sides incline more laterally. In the interparietal 

 region it is of less breadth and capacity than in the Muskrat, and in the frontal 

 region is wider and more capacious. 



A narrow sagittal crest separates the temporal fossae, and extends from the inion 

 to near the middle of the frontal bone, and to the most constricted portion of the cra- 

 nium, before it bifurcates to define the lateral borders of the forehead. The temporal 

 surface is extensive, and reaches forward upon the side of the frontal bone in ad- 

 vance of its middle. 



The interparietal bone is shield-shaped in outline, nearly straight behind, sub- 

 angular and without prolongation in front, and slightly incurved at the sides. 



The interparietal suture appears as a fissure dividing the sagittal crest. 



The temporal bone contributes but a small proportion of surface to the temporal 

 fossa. The temporo-parietal suture pursues an irregularly horizontal course backward 

 to the boundary of the inion. The upper border of the temporal bone is pierced by a 

 foramen, in the line of the temporo-parietal suture, as large as that existing in the 

 Muskrat. 



The frontal bone, in the specimen, is divided by a median fissure, apparently a con- 



