100 Mr. Bnooxzs on a new Genus of the Order Rodentia. 
may be noticed in the squirrel and Pteromys. The toes are 
three, the middle one being the longest and the inner one the 
shortest. | 
From this the hinder extremity of Dipus differs most essen- 
tially. Its femur is arched, with the convexity forwards, and 
is only half the length of the tibia. The fibula is short, and 
extends to just below the middle of the tibia, where these 
bones are firmly ossified together: the metatarsal bone, which 
is about two-thirds of the length of the tibia, is single, and is 
terminated by three nearly equal toes, the lower part of the limb 
bearing in the skeleton a striking resemblance to that of a small 
tridactylous wader. ; 
. To this outline of the more remarkable particulars exhibited 
by the skeleton of Lagostomus, and of the numerous and im- 
| portant differences which exist between it and that of the Jer- 
boa, a few observations may be added respecting its relation 
with those of other rodent quadrupeds nearly approaching to it 
in size. 
The Lagostomus has 12 ribs, and consequently twelve dorsal 
Jerboa. . 12 
Squirrel . . 12 —— 
Rabbit . « 12 —— 
Marmot... -JS —— 
Coypus . . 13 —— 
[ vertebrae, 
Apouu —. «43 
Urio .-...14 
Capromys. . 16 —— 
The Lagostomus . 7 lumbar vertebra. 
JUD. . XY 
Squirrel . . 7 — 
The 
