PETERSON ; DESCRIPTION OF NEW RODENTS 163 
broad as that in the beaver. As a whole, the caleaneum is perhaps more similar to 
that of Castor than to that of Cynomys or Aplodontia. 
Astragalus. —The principal difference, in the astragalus of S. fossor and Castor, 
is found in the prominent tuberosity below the articulation for the tibia, on the 
internal side near the sustentacular facet in the latter. This tuberosity is not present 
in S. fossor. Cynomys has it slightly indicated. In Aplodontia it is like that of S. 
fossor. In the latter, the constriction above the navicular facet, is relatively longer 
than in Castor, and is similar to that in Cynomys. The articulation for the tibia is 
regularly convex antero-posteriorly, and concave transversely, forming a rather shal- 
low trochlea, similar to that in Cynomys. The navicular articulation is less spherical 
in the latter genus than in S. fossor. 
The Cuboid. —The cuboid is injured antero-superiorly, so that the articular sur- 
face for the calcaneum is nearly lost, in the type. Judging, however, from the 
rounded, shallow pit on the distal end of the calcaneum, the facet on the cuboid 
would have a correspondingly convex, rounded appearance. In the beaver, the 
ealeaneal articulation is a long, narrow, convex surface, which is placed obliquely 
antero-externally and postero-internally on the proximal face of the bone. The 
cuboid in the type has a more depressed appearance than in Cynomys, the distal 
end being more expanded, with a larger articulation for mt. 1V., and no constriction 
above this facet on the posterior face of the bone, as in Cynomys. There is a strong 
contact for the ectocuneiform and the nayicular. 
Metatarsal I. —(Pl. XVIIL., fig. 14.) The first metatarsal is somewhat more re- 
duced in length than that of Cynomys and Aplodontia, and is not at all like the 
greatly developed hallux of the beaver. The digit is parallel with the long axis of 
the pes, as in Cynomys and Aplodontia. The proximal articulation is long and 
irregular antero-posteriorly, and narrow transversely. The ascending postero-external 
process, which reaches over and articulates with the plantar face of mt. II. in Cyn- 
omys, is represented only by a short rounded knob in S. fossor ; it extends directly 
backward. he articulation for mt. IL. is quite strong. The distal end is slightly 
enlarged, and the shaft consequently constricted ; the transverse diameter is a little 
greater than the antero-posterior. 
Metatarsals IT. and ITIL, in the type, are of nearly equal length. Mt. II. has a 
narrow proximal articulation, and not an expanded head like that in Cynonvys. 
The shaft is flat and depressed, distally enlarged, and has a heavy rounded plantar 
keel. The second digit in the beaver is comparatively lighter than in S. fossor; in 
Cynomys and Aplodontia, this condition is more nearly similar to that in the fossil. 
Mt. ILI. is the strongest metatarsal in the type; there is, however, not much differ- 
