96 Mr. Brookes on a new Genus of the Order Rodentia. 
author, indeed, appears to have doubted the propriety of this 
location, and mentions with evident regret, that the loss of the 
remains of the animal had prevented our becoming acquainted 
with its organization, and ascertaining precisely its characters. 
Fortunately, however, the animal, although obscured from notice 
during so long a period, is yet in a condition for accurate and 
minute examination. 
_ The description of its preserved aii and skeleton I have now 
the honour of presenting to the notice of the Society ; and from 
the structure of the latter especially, it will be evident that it 
must be referred to a new genus, to which I propose to give the 
name of Lagostomus. The form of the teeth, on which so much 
stress is justly laid in characterizing genera, differs essentially 
from that exhibited by all the other Rodentia ; from which it is 
also distinguished not only by the number of its toes, but by 
various other particulars of its osteology, which I shall now 
proceed rapidly to describe, assuming occasionally as a point of 
comparison the skeleton of the Dipus Sagitte, with which it has 
been generically confounded. 
The upper surface of the cranium in Lagostomus exhibits the 
usual form of that of the Rodentia, its sides being nearly pa- 
rallel, and its occipital breadth scarcely exceeding its breadth 
immediately anterior to the orbits. In Dipus, on the con- 
trary, the outline is decidedly triangular, arising from the very 
considerable dilatation of its hinder part, occàsioned by the 
extraordinary development of the mastoid processes of the tem- 
poral bones, which are extremely delicate, and possess, as in 
man, a cellular structure*. The 
* The Egyptian Jerboas being known to domiciliate themselves under bushes fre- 
quented by the Cerastes, so that it frequently, or perhaps generally occurs, that where . 
the one, there the other is also found ; this particular osseous extension may be destined 
by Nature, to give increased sensitiveness to the auditory organ, for the greater security 
In the Chlamyphorus truncatus there are two somewhat similar osseous 
tumours 
of the animal. 
