178 THE WASATCH AND REIDGEU FAUN^. 



of the SciuridcE. The great trochanter is about as high as the head, and 

 bounds a large fossa. The little trochanter is well developed. The rotular 

 groove is moderately elongate, with nearly equal bounding keels. The 

 condyles are subequal and present posteriorly. The distal extremity of 

 the tibia supports an internal malleolus which is flat on its inner side, and 

 is without distal facets. The external trochlear groove of the peronealy is 

 larger than the internal. The posterior border is produced downwards in 

 a subangular process as in other iSciurida; which is as long as the malleolus, 

 and is openly grooved to carry the tendon of the flexor longus polUcis mus- 

 cle. It is separated by a deep notch from the internal malleolus, through 

 which passed the tendons of the flexor longus digitorum, and the tibialis pos- 

 ticus muscles. 



The astragalus also resembles that of a squirrel. The head is du-ected 

 inwards from the anteroposterior axis of the troachlea, and has a depressed 

 and convex distal extremity. The trochlea is wide, and the groove is well 

 marked. Its external and internal faces are vertical. The calcaneum is 

 large, and the free portion is compressed. The sustentaculum is small. 



The species from which most of the characters of the genus as above 

 stated have been derived are the P. delicatior and P. delicatissimus. They 

 display the following general points. The. anterior limbs are relatively 

 longer than in recent species of squirrels. The bead of the radius is rounder, 

 indicating an unusual power of rotation of the anterior limb. The pelvis 

 is larger, being as long as the skull, and it is probable that the posterior 

 limb is larger. These points indicate approximation to the Mesodotda. 



No characters have yet been oflfered by which to distinguish the Ameri- 

 can species as representing a genus distinct from the Plesiarctomys gervaisii 

 of the French Eocene. Bravard briefly distinguishes the genus as distinct 

 from Ardomys in the greater thickness of the angles of the molars, which 

 thus become tubercles. Only the mandible and mandibular teeth of the 

 P. gervaisii are known. It has been idund in thf Tppi r Eocene, near 

 Perreal, Apt, France. 



I have seen five species of this genus, of which one, P. liiaua belongs 

 to the Bridger beds ; one, P. leptodus, to the Washakie ; one, P. buccafus, to 



