154 MEMOIRS OF THE CARNEGIE MUSEUM 
flattened, expanded distally, and provided with a strong contact for the cartilag- 
inous rib of the sternum. The shaft of the rib decreases in width superiorly, and 
is nearly round immediately below the heavy tuberculum. ‘The head is rather 
small and rounded. The rib as a whole is different, especially distally, from that 
of Cynomys and Aplodontia. The latter genera have the lower end of the rib more 
rounded in cross section. 
The ribs of the anterior half of the thorax are flat proximally, and rod-like dis- 
tally. In the meso-thorax, and more posteriorly, they are rod-like throughout. 
The Clavicle. —The clavicle in 8. fossor is heavier than in either Cynomys or Aplo- 
dontia. In the cotype (No. 1208) both clavicles and the manubrium are present. 
The latter is much heavier than in Aplodontia or Cynomys. Anteriorly, there is a 
broad, gently rounded surface for the attachment of the clavicles. The attachments 
for the first set of ribs are very large, and are located close to the anterior end of the 
bone. Back of this rib-attachment is a long and gentle Jateral constriction. Infe- 
riorly, the bone is slightly keeled ; supero-anteriorly itis concave. Posteriorly it 
expands above and on the sides, indicating a rather large mesosternum. 
Both clavicles in the type are complete, and are not so robust as those of the cotype. 
The latter specimen represents a somewhat larger and more robust individual. In 
shape, the clavicle is very similar to that of the beaver. The sternal attachment is 
more enlarged than in Cynomys and Aplodontia. ‘The curve of the shaft in S. fossor 
is similar to that in Aplodontia and Castor. The superior end is less flattened than 
it is in the beaver and the prairie-dog, and does not terminate in an enlarged head 
as in Aplodontia. ‘The clavicle of S. fossor gradually tapers from the large sternal 
contact to the superior end, which has on the internal face a wide, shallow groove 
at the contact with the acromion process of the scapula. 
MEASUREMENTS. 
Approximate length of the vertebral column including skull.........................eseeeeeeee 37 cm. 
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Greatest antero-posterior diameter of atlas..............0.:ccccseceecceceeenesec eee eeeeceeeeeeeeeseeees eee 
Greatest transverse diameter Of atlas..................cccecceceeeeeteeeeeneereeeeeeeeesetenseceeseee sees OB} 8 
Greatest diameter of cotyli for occipital condyle... ................ceceeeeeceeceeceeceeer eee eeeeeeeens Sins 
Greatestsheich tioffatlastresssn-cscsecusste seine pees sotecceeseeciaeetcseceeceeactecreceeaecneerases Ie % 
Antero-posterior diameter of centrum including odontoid process of axis g 
Antero-posterior diameter of centrum of axis at the base of the odontvid process........... Bye Oy 
Greatest transverse diameter of axis anteriOrly................cceceece sce eec eee eee eeeseseeneseeeeeees 122 66 
Greatest height of axis including spine.......... .......0ccecceec eee eec eee eee eee eeeeecseueereceeeseeeees 1G 
Greatest height of neural spine of axis. Gres 
Antero-posterior diameter of third cervical acy 
a sf se fourth ‘“ 33) 
. ae ee fifth ee 3.5 ‘f 
