oe. a ee PLDT as hae 
Mr. Bnooxzs on a new Genus of the Order Rodentia. 97 
The occipital spine in the Jerboa is very trifling: a singular 
circumstance, when considered in connection with the upright 
position continually assumed by that animal; while in Lago- 
stomus it is most strongly and decidedly pronounced. In both, 
the orbit is separated from the temporal fossa by a broad and 
strong bony process ;—a structure which occurs also in Echi- 
nothrix dorsata, and in Celogenus Agouti; but not in Lepus, 
Arctomys, Castor, and many others of the Rodentia. The zygo- 
matic arch is weak posteriorly. | 
The incisor teeth, as in most of the genera of this order, are 
two in number in each jaw; they are long, and protrude con- 
siderably, almost equalling in this respect those of Orycterus 
maritimus, and exceeding those of any other species, with that 
exception: those of the lower jaw are rather the longest, and 
are grooved along the middle line of their outer surface. The 
molar teeth are four in number on each side of each of the 
jaws: those of the lower jaw are placed in a very oblique 
direction forwards and outwards; each of them is composed 
of two equal portions, distinctly surrounded by a margin of 
enamel, and closely united, so as to give the appearance 
of two single flat teeth intimately ossified together laterally. 
The three anterior molar teeth of the upper jaw very much 
resemble those of the lower, but are placed somewhat less 
obliquely : the fourth, or hinder one, differs in having added to 
it a third portion, which is rather smaller than the others, and 
is rounded in its posterior outline. In the Jerboa the molar 
teeth, it is almost unnecessary to remark, are only three in num- 
ber on each side of the lower jaw: the structure of their crowns, 
in which the circumvolutions of the enamel are so complicated 
tumours situated just above the orbits, the use and connections of which are unknown, 
but which may be probably intended for an extension of the olfactory organ, or possibly 
for a more elaborate diffusion of sound in its subterranean pursuits. 
VOL. XVI. o == HS 
