152 MEMOIRS OF THE CARNEGIE MUSEUM 
The First Lumbar. — The first lumbar vertebra is decidedly heavier than the last 
dorsal. It is further characterized by the sudden increase of the antero-posterior 
diameter of the neural spine. If the last dorsal and the first lumbar had not been 
found in position, I would have been inclined to think that there might be a vertebra 
missing at this point. This vertebra resembles the preceding in not having a trans- 
verse process. Otherwise this bone is very similar to the following vertebra, which 
has all the characteristic features met with in this region. ‘The inferior surface of 
the centrum is slightly keeled, and has a greater diameter antero-posteriorly than 
the last dorsal. The prezygapophysial articulation is a large surface, and is slightly 
concave. 
The Second, Third, and Fourth Lumbars are so similar to one another that the 
description of one will answer for all. The antero-posterior diameter of the centra 
gradually increases; their inferior faces are more keeled; and the transverse 
processes become longer from the second to the fifth lumbar. The fourth lumbar 
has apparently no metapophysis. In this respect it is similar to the same vertebra 
in Aplodontia. The transverse processes are short and heavy, and are located on 
the pedicle, and not directly in contact with the centrum asin Cynomys. In Castor, 
these processes are high, but not as high comparatively as in S. fossor and Aplodontia. 
The Fifth Lumbar. — This vertebra is represented by the complete neural arch, 
the centrum having apparently become absorbed by plants that are so common 
inside of Demoneliz. The most characteristic feature of this vertebra is the sudden 
decrease of the neural spine. This is more apparent than in either Cynomys or 
vplodontia. The transverse process is present on the left side, and is situated on a 
horizontal line with the postzygapophysis. 
The Siath Lumbar. — In the type, the left postzygapophysis and the end of the 
right transverse process are missing from this vertebra. ‘The centrum has a greater 
antero-posterior diameter, and is more keeled than in any other presacral. The 
transverse process is prominent, and situated high as in the preceding lumbars. 
The neural spine is somewhat more reduced than that in Aplodontia. Otherwise 
this vertebra is very similar in these two genera. 
The Sacrum. — There are five well codssified centra in the sacrum of S. fossor. 
In Aplodontia there are six, in a young Cynomys three, and in Castor four. In the 
cotype (No. 1208) there are five lumbar vertebree and a sacrum; the last sacral is 
lost, but the centrum and the pleurapophyses on the preceding vertebra show, that 
there was coossification between the fourth and fifth sacrals. The number of sacral 
vertebrae in S. fossor consequently seems to be five, at least in two distinct cases. 
The centrum of the anterior sacral vertebra in the type (No. 1217) is very nearly as 
