PETERSON : DESCRIPTION OF NEW RODENTS 159 
pollex represented in the type. Another individual of the same species, which is more 
robust (No. 1204) has the phalanges, first, second and third metacarpals, and the 
unciform preserved... A composite figure of the two specimens, the type, No. 1217 
and No. 1204, represents the left manus of S. fossor on PI. XVIII, fig. 5. The 
slight disparity of the two individuals is at once noticed in this illustration. 
The Unciform in the type supports the fourth and fifth metacarpals. This bone 
is rather broad laterally, and the articulation for the scapholunar and cuneiform, is 
postero-radially a higher ridge than in Aplodontia, and is more nearly like that of 
Cynomys. . Distally the articulation for the metacarpals is triangular in outline ; it 
is concave fore and aft, and slightly convex transversely. The bone as a whole is 
similar to that in Cynomys. 
The Trapezoid. — The trapezoid is a small irregular bone, which supports me. IT. 
The inferior articulation is obliquely concave antero-posteriorly. The superior sur- 
face is nearly flat. 
Metacarpal I.—The first metacarpal is fairly well developed, and is parallel 
with the long axis of the manus. The first phalanx is strong, and supports an 
ungual, which is perhaps moderately developed as in Cynomys, Aplodontia, and other 
genera of this family. There is what appears to be a radial sesamoid in position 
in a smaller species. ‘This species will be described later on in this paper. 
Metacarpal IT. is shorter than in Aplodontia, and is of about the same size. 
Metacarpals III, IV., and V. are present in the specimen No. 1204, and are 
relatively short and heavy. Me. III. is broken off superiorly, but it indicates a bone 
as long as the corresponding one in Aplodontia, and is much heavier. In the fossil, 
me. III. is in comparison somewhat more expanded distally than that of Aplodontia. 
Me. IV. is of the same length as that of Aplodontia, but is much heavier than in this 
genus or in Cynomys. In our fossil, the proximal articulation is deep antero-pos- 
teriorly. This is due to the large tuberosity on the plantar face of the bone. The 
shaft is slightly constricted, and the distal end has a strong keel on the palmar face. 
Metacarpal V.is extremely short, has practically no shaft, and is as heavy as me. 
IV. The articulations for the unciform and the proximal phalanx are quite large 
on the nodular-shaped bone. ‘There is a deep transverse groove on the plantar face. — 
The proximal row of phalanges are depressed and transversely broad. The proxi- 
mal articulation is a rounded pit with a moderately deep notch on the inferior 
margin. The second row of phalanges are even more depressed than the proximal 
row. Proximally, the phalanges are concave antero-posteriorly and slightly convex 
transversely. The distal trochlea is broad, flat, and extends to a slight degree on 
the anterior face of the bone. 
