62 MR. E. T. BENNETT ON THE CHINCHILLID&. 
sides other minute differences in these organs, exhibit considerable discrepancy in the 
form of the skull and in various particulars of their organization, into which I may 
perhaps enter more fully on a future occasion. 
At no great distance from these, if we are to judge alone by the characters of its 
molar teeth as figured by M. F. Cuvier, is placed the genus Helamys, of which unfor- 
tunately I possess neither skull nor skeleton for comparison. As far, however, as the 
stuffed skin can be relied on for such a purpose, it seems in many of its characters to 
approach Lagostomus ; while the differences in the relative proportion of its limbs, in 
the elongated claws of its anterior extremities, in the character of its fur, and above all 
in the structure of its teeth, forbid a close approximation. With no better materials 
than I can at present refer to, it would be hazardous to attempt to assign its true 
position. 
Equally anomalous appear, in the present state of our knowledge of the tribe, two 
North American genera, first established by Dr. Rafinesque, Geomys and Diplostoma ; 
with which must be associated Aplodontia of Dr. Richardson. The teeth of Geomys and 
Aplodontia, as figured by the last-named zoologist, are more simple in their structure 
than those of any other genus of Herbivorous Rodentia; and, in so far, they seem to 
approach the groups of which I have just been speaking. But the discrepancies in 
other respects are so considerable, that further information must still be considered 
necessary to determine their real affinities. 
To the Arvicolide succeed the Caviide, comprehending Cavia, Kerodon, and Doli- 
chotis (Cavia Patachonica, Shaw) ; in which the dental triangles are more distinct and 
elongated, and form but one series instead of two, their acute angles projecting ex- 
ternally in the lower jaw and internally in the upper: they are also fewer in number. 
The number of the molar teeth is four; and here again, as in the Hares and Rabbits, 
their crowns are not directly opposed to those of the opposite jaw. The order is in 
these, however, the reverse of that which occurs in the genus Lepus, the teeth of the 
upper jaw being received within those of the lower; and a degree of obliquity is given 
to the insertion as well as to the surfaces of the teeth, sufficient to produce the effects 
of a more perfect opposition. 
The series is closed by Hydrocherus, which exhibits, in its dental character, a still 
further complication of the structure observed in Arvicolide and Caviide, with much 
that is aberrant in the order. Some valuable observations on this point by Mr. Owen 
have been published in the ‘Proceedings of the Committee of Science and Corre- 
spondence’ of this body!?. 
The mere enumeration of the groups of this interesting little tribe may serve as an 
illustration of the advances which the science of Zoology has made within a very few 
years. Of the nineteen genera named, no less than eleven (including the whole family 
of Chinchillide, Capromys, Ctenodactylus, Octodon, Geomys, Diplostoma, Aplodontia, 
1 Part II. p. 187. 
