MR. E. T. BENNETT ON THE CHINCHILLID®. 6) 
excisure of the posterior margin of the lower jaw, which in the latter animals is an ex- 
tremely broad and rounded bony plate ; and many other equally striking, but less im- 
portant, modifications in the bony structure of the head alone. In the possession of 
perfect clavicles, and the substitution of a large open suborbital passage for the perfo- 
rated bony plate found below the orbit in the Hares and Rabbits, they exhibit a more 
immediate affinity to the genus Lagomys, in which both these modifications are stated 
to occur, 
Most nearly related to the Chinchillide on the opposite side is the genus Capromys, 
Desm., (Isodon, Say,) in which the molar teeth (also four in number on each side, with 
flattened crowns and surfaces exactly opposed to each other,) would appear on a super- 
ficial observation to be subdivided in a nearly similar manner. On looking closer, 
however, we observe that the processes of enamel do not, in Capromys, entirely traverse 
the teeth, but form alternate indentations, corresponding with external sulci, of which 
two are seen on one side of the tooth, and one only on the other. The anterior teeth 
are also more elongated, and that of the lower jaw projects forwards in an angular 
process, which has on its inner margin a third denticular fold of enamel. The cranium. 
in Capromys is very narrow and much elongated, especially in its frontal and parietal 
portions, the latter having a slight convexity at its anterior part ; the orbit is of small 
capacity ; and the infra-orbital foramen is large and open, as in the Chinchillide. The 
number of the ribs, which is no less than sixteen, is here a marked peculiarity ; and. 
the great strength of the limbs, supported, as regards the anterior, by a complete cla- 
vicle, and having the bones of the fore-arm nearly equally developed, and the jibula 
strong and distinct, affords a characteristic difference, connected doubtless with the 
scandent habits of the animals. 
The peculiarity noticed in the anterior tooth of the lower jaw in Capromys leads us 
by a natural transition to the typical Arvicolide, including Arvicola, Lemmus, and Fiber, 
in which only the three anterior molar teeth are developed. These have their enamel 
so folded inwards as to form on both the outer and inner side of each tooth a series of 
distinct triangles alternating with each other, and giving rise to acute-angled projections 
both externally and internally. In the genera of this family the increasing rounding of 
the anterior surface of the lower incisors renders their points, when obliquely worn 
down, either rounded or acute, instead of transversely truncate, as in the preceding 
groups. 
Nearly related to Arvicolide, with which they agree in general appearance and in 
mode of life, are two genera, at present standing in some degree isolated, Ctenodactylus! 
and Octodon®. The resemblance in the form of the molar teeth in these two curious 
animals, (both of which have been very recently for the first time described,) one from 
Africa and the other from Chili, is very remarkable. They differ, however, in number. 
the latter having four and the former three only on each side of either jaw; and, be- 
' Gray, Spicilegia Zoologica, p. 10. t, 10. . * Bennett, in Proc. Comm. Sci. Zool. Soc., Part II. p. 46. 
