PETROMYS TYPICUS. 
there is a small tuft of bristles, similar to those forming the whiskers. Legs 
rather short ; the soles of the fore feet, and the tarsi of the hinder ones, bare, 
and of a brownish-black colour. The toes are rather short ; and, as well as 
the claws, fully covered with fur ; the two middle ones of each foot nearly of 
equal length, and slightly longer than the lateral ones. Claws short, slender, 
and slightly curved. Tail slender, cylindrical, and every where fully covered 
with rigid hairs ; those towards the tip, much the longest. The fur of the head 
and body, is rather long and recumbent, closely set, and rather harsh. 
The skull posteriorly very broad ; above, between the hinder edges of the 
parietal and the apices of the nasal bones, flat. The parietal bones are divided 
by a longitudinal suture, as are also the frontal ones ; the latter are very 
broad, so that the breadth between the eyes is unusually great, when com- 
pared with other rodents of the same size. The orbits are rather small, and 
their circumference, internally, irregular and broken. The malar bones are 
well developed, somewhat triangular in shape, and each externally with an 
obtuse carina along its middle; the point of the ti’iangle joins the zygomatic 
process of the temporal bone. Infraorbital foramen very large. Tympanitic 
Indies large, subglobular, and semi-transparent. Foramina incisiva double. 
The incisor teeth of the upper jaw are long, and semicircular ; the basal two- 
thirds concealed in the intermaxillary bone ; the apex of the exposed third of 
each, with a sharp cutting edge anteriorly ; behind which is a deep notch, 
formed to receive the point of the corresponding tooth of the lower jaw. The 
incisors of the lower jaw are laterally compressed, and more wedge-shaped at 
the points, than those of the upper jaw. The molars of both jaws are eight 
in number, four on each side, of nearly the same form in both, and without 
true fangs. They are all nearly of the same size ; and the crown of each is 
crossed by two transverse fosses, formed by the projecting enamel. The 
margin of the crown externally entire, and arched ; internally notched ; — and 
where this notch occurs, the enamel, with which it, as well as the rest of the 
tooth is edged, sends off, in the teeth of the upper jaw, a thin lamella, which 
proceeds nearly across the middle of the crown of each, in its course dividing 
and leaving a minute oval opening, about half-way between the notch and 
outer edge of the tooth. In the teeth of the lower jaw, neither the transverse 
lamella; of the enamel, nor the openings, are visible. The rami of the lower 
jaw, posteriorly, or those parts which, when they ascend, are distinguished 
by the name of ascending rami, highly divaricated. The superior coronoid 
process delicate and very small; the inferior one long, pointed, projecting 
behind the line of the articular process. The configuration, &c. of certain of 
the abdominal viscera, will be understood from an examination of Plate XXI. 
