162 MEMOIRS OF THE CARNEGIE MUSEUM 
the articular surface has apparently a greater antero-posterior convexity, indicating 
perhaps a greater flexion of the limb than in Cynomys or Aplodontia. In the type 
the median eminence, or spine, is well developed, showing strong attachment for an 
intra-articular ligament. On the fibular side, near the edge of the articulation, 
there is a small sesamoid in position, similar to that seen in Aplodontia on the 
articular surface of the tibia. Proximally the shaft is strongly trihedral. The 
cnemial crest is as strong as in Aplodontia and S. penimsulatus. In the latter species, 
the tibia and the femur are of more nearly equal length than in S. fossor.’ 
Distally the shaft is more rounded in cross section. The astragalar facets have 
a somewhat greater antero-posterior diameter than in Cynomys, and the posterior 
downward process or malleolus is strong and not grooved as in the recent genus. 
The external articular facet for the astragalus is higher up on the bone, has an oblique 
position, and is much larger than the internal facet. There is a more distinct ridge 
separating these two facets in S. fossor than in Cynomys. 
The Fibula. — The fibula is relatively as large as that of Aplodontia and Cynomys, 
and, as in these genera and the beaver, it is free. The proximal end is somewhat 
injured in the type, but it indicates a large tuberosity on the external side for the 
attachment of the lateral ligament, such as is found in Aplodontia and Cynomys. 
In Castor, this tuberosity is produced into a prominent process, directed downward 
and outward. In Fiber zibethicus this process is also quite prominent. The shaft 
of the fibula, in the type, is more flattened superiorly than in Aplodontia, and is 
more nearly like that in Cynomys. The distal epiphysis is slipped off so that its 
character cannot be ascertained. The caleaneum indicates, however, that the fibula 
may have touched the exterior face of the sustentacular facet. 
The Tarsus. —The tarsus is represented in the type by the calcaneam, astragalus, 
cuboid, all the metatarsals, and the phalanges of the third and fourth digits. The 
second phalanx of the second digit is also present. 
The tarsus and carpus of S. fossor have approximately the same relative size as 
in Aplodontia and Cynomys, and are entirely unlike those in Castor, as the following 
description will show. 
Caleanewm. —'The caleaneum in S. fossor has, as in S. monianus, “a short de- 
pressed, irregular, and club-shaped tuber.” The tuber is relatively much broader 
than in the beaver, but the oblique exterior face seen in the latter genus, when the 
bone is in its position in the pes, is also very apparent in S. fossor. The articulation 
for the cuboid is a rounded, shallow pit. ‘The sustentacular facet is relatively as 
1Piofessor Cope’s illustration (‘‘ Tertiary Vertebrata,’’ P]. 63, fig. 21) is incorrect. This illustration indicates a part 
of the shaft missing, but the true contact between the two parts has been found, and the bone fitted together, in the 
American Museum of Natural History. 
