Mr. Brookes 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 Ccelogenus 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 Orycteriis 

 maritiinus, 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 as 



