834 THE WHITE EIVEE FAUNA. 



The section of the ilium at its base and at its middle, is triangular. 

 There is an angle along the middle of its external face which supports a 

 moderately prominent tuberosity, a little above the acetabulum. On the 

 anterior mai-giu a little higher up, is a short, compressed, rather prominent 

 process, whicli j^robably represents the anterior inferior spine. From this 

 point posteriorly the internal face of the ilium is deeply concave, producing 

 an attenuation of the inner wall of the acetabulum. The ischium is rather 

 narrow at the base. 



The distal portion of the tibia is much like that of Ardomys, Gymnop- 

 tyckus, and other Sciuridce. The posterior median process is very large and 

 is shallowly grooved ; the usual deep tendinous groove separates it from the 

 internal malleolus. The trochlear grooves are deep and well separated ; 

 the fibular surface is short. The head of the astragalus is horizontally oval, 

 and is separated from the trochlear portion by a neck of moderate length. 

 Ii extends obliquely inwards, so that the internal margin of the head is in- 

 terior to the line of the inner margin of the trochlea. The sides of the latter 

 are vertical. It is considerably wider than long ; the trochlear carinae are 

 marginal, and the external is considerably more elevated than the inner. 

 The separating groove is profound but open. The posterior inferior fossa 

 is small and foramen-like. 



Besides the very different form and position of the infraorbital foramen, 

 this genus differs from Gymnoptychus in the excavation of the posterior edge 

 of the palate. 



Dr. Leidy remarks that this genus belongs to the family of the Sciuridce. 

 This is indicated by the dental characters ; but in some other respects there 

 is a greater divergence from the squirrels and marmots than is the case with 

 the preceding genus, Gymnoptychus. Thus, the large foramen infraorhitale 

 anteritis occupies the elevated position at the origin of the zygomatic arch 

 seen in the porcupines and cavies. There is no superciliary ridge nor post- 

 orbital process as in most Sciuridce, but the front is contracted between the 

 orbits in the same manner as, but to a less degree than, in Fiber, and the 

 Eocene Plesiarctomys, Brav. Both the last named and Ischyromys present 

 many points of resemblance to Pomel's tribe of Protoniyidce, but differ from 

 any of the genera he has included in it. 



