PETERSON : DESCRIPTION OF NEW RODENTS 151 
compressed. The tips of the neural spines of the sixth and seventh dorsals are 
broken off. 
The Tenth Dorsal. — In the type, this vertebra is only represented by the neural 
spine, the prezygapophyses, and a small portion of the anterior part of the arch. 
This vertebra has the most delicate spine in the dorsal series. 
The Eleventh Dorsal. —'The eleventh dorsal is distinctly different from the pre- 
ceding dorsals. This difference is chiefly found in the lateral convexity of the 
postzygapophyses, and in the presence of well developed metapophyses. The trans- 
verse process is only a small rounded knob, situated immediately below the base of 
the metapophysis on the exterior face of the pedicle. ‘The spine of this vertebra is 
much higher than on the corresponding vertebra in either Aplodontia or Cynomys. 
The Twelfth Dorsal. —There is no transverse process on this vertebra. The 
metapophysis is broken off. The neural spine is heavier, but is not so high as in 
the preceding vertebra. The postzygapophysis is more rounded than in the eleventh 
dorsal, and its articular face is directed more outward than downward. ‘The cen- 
trum is less depressed than in the preceding vertebree. 
The Thirteenth Dorsal. —Vhe right prezygapophysis and metapophysis are broken 
off from this vertebra in the type. The top of the neural spine is also injured. The 
centrum is much concave fore and aft, and is very little longer than that of the pre- 
ceding vertebra. The large postzygapophysial articulation faces downward and 
outward, but is not as convex as that of the eleventh and twelfth dorsals. The strong 
metapophysis takes its origin at the base of the prezygapophysis on the superior part 
of the pedicle and terminates in a rounded point in a parallel line with the postzy- 
gapophysis. Its size, shape, and position are very similar to those of Aplodontia and 
the prairie-dog. 
The change of direction of the neural spine occursin the tenth dorsalin Cynomys, 
while this change of position is only slightly noticeable in the tenth and eleventh 
dorsals of Aplodontia. In Steneofiber fossor the change of direction of the neural 
spine is gradual, similar to that in Aplodontia, but it takes place in the twelfth and 
thirteenth dorsal (as in Castor), instead of the eleventh and twelfth. S. fossor also 
differs from Aplodontia and Cynomysin having higher spines on the posterior por- 
tion of the dorsal region. 
The Lumbar Vertebrx. —'The three anterior lumbars and the thirteenth dorsal 
were found articulated with one another by their zygapophyses. These vertebree 
are complete except some of the delicate processes. As a whole, the lumbar 
region in S. fossor is more nearly similar to that of Aplodontia than to that of 
Cynomys or Castor. 
