PETERSON : DESCRIPTION OF NEW RODENTS 157 
metacromion, and the broad acromion process, is very similar to that of Castor can- 
adensis. 
The Humerus. — Both humeri are represented in the type as well as in the cotype. 
This bone (P]. XVIII, fig. 3) is comparatively short and stout, and has a great lateral 
expansion distally. Proximally, the large head is well rounded, the antero-posterior 
diameter being the greatest. ‘The tuberosities are fully as large as those of Aplo- 
dontia. The shaft is triangular in cross-section, especially so superiorly. The deltoid 
ridge is strong, with a transversely placed broad plate. This plate nearly equally 
overhangs the ulnar and radial sides of the deltoid ridge ; it is entirely different from 
that in Cynomys, Aplodontia, or Castor; in the latter it has a large tuberosity on this 
ridge for the attachment of the deltoid muscle. In the type of S. fossor on the ulnar 
side below the tuberosity there is a short, sharp ridge, turned slightly posteriorly. 
As has been stated, the humerus is greatly expanded distally, the large internal epi- 
condyle and the greatly expanded supinator ridge being the chief factors in this 
expansion. As in Stencofiber montanus, the supinator ridge extends high upon the 
posterior aspect of the shaft. The trochlea is relatively as broad and as shallow as 
that in the beaver. Cynomys has the trochlea much deeper, and Aplodontia has it 
about the same depth as S. fossor. The supratrochlear fossa isas deep or deeper than 
in Aplodontia, and the anconeal fossa is approximately the same as in that genus. 
The internal epicondyle is comparatively heavier than in the beaver, and is perfor- 
ated by a large foramen, as in S. montanus and in Aplodontia. In the mole the 
internal epicondyle is relatively heavier than in S. fossor, and is perforated by a 
foramen. 
radius and Ulna.—The right and left radius and ulna are present in the type 
(Pl. XVIITI., fig. 4,4a@). The forearm is relatively shorter than in Cynomys or Aplo- 
dontia. The shaft of the radius is a round and irregularly curved rod, and may be 
in somewhat closer proximity to the shaft of the ulna than is represented in our 
illustration. Superiorly the radius flares out abruptly into a large head to furnish 
a good support for the trochlea of the humerus. Distally. the radius is even larger 
than proximally, and sends down a heavy internal and a smaller external process 
below the large articular facet for the carpals. In S. fossor this external process is 
much better developed than in Cynomys, Aplodontia, and the beaver. 
The ulna has a heavier shaft than the radius; it is compressed laterally, and is 
broad antero-posteriorly, with a shallow groove extending from the sigmoid notch 
to near the distal end of the bone on the external side. This groove is better defined 
than in Aplodontia, which genus has the bones of the forearm in close contact 
throughout. In Cynomys the radius and ulna are separated, as in S. fossor. In 
