MR. E. T. BENNETT ON THE GENUS OCTODON. 77 
of the form of the molar teeth of Octodon, and by a comparison of them with those of 
the allied genera. 
The upper molars of Octodon have, on their inner side, a slight fold of enamel, in- 
dicating a groove which has a tendency to separate, on this aspect, the mass of each 
tooth into two cylinders. On their outer side a similar fold penetrates more deeply, 
and behind it the crown of the tooth does not project outwardly to so great an extent 
as it does in front. If each molar of the upper jaw were theoretically regarded as com- 
posed of two cylinders of bone, surrounded by enamel on all their aspects except that 
by which they are broadly united to each other, slightly compressed from before back- 
wards, and somewhat oblique in their direction as regards the axis of the jaw, the an- 
terior of these cylinders might be described as being entire, and the posterior as being 
truncated by the removal of its outer half. Of such teeth there are in the upper jaw 
of Octodon, on each side, four; the hindermost being the smallest, and that in which 
the peculiar form is least strongly marked. In Ctenomys the molar teeth, both of the 
upper and lower jaw, correspond with the structure that exists in those of the upper 
jaw of Octodon. They are formed on precisely the same type. The exceptions to their 
perfect similarity consist in their crowns being slenderer and more obliquely placed, 
whence their emargination becomes less sharply defined ; and in the hinder molar of 
each jaw being so small as to be almost evanescent, and consisting of a single minute 
triangular prism. As is generally the case, however, in the dentition of this tribe of 
Rodents, the relative position of the teeth is in Ctenomys counterchanged in the two 
jaws ; and the vacancy in the outline of the crown of the molars, which in the upper 
jaw is external and posterior, becomes in the lower jaw internal and anterior. 
In the lower jaw of Octodon the crowns of the molar teeth assume, as has been 
already remarked, a figure very dissimilar from those of the upper, dependent chiefly 
on the prolongation of both portions of the tooth to the same lateral extent, and on the 
depth to which they are penetrated on their inner side by the fold or emargination be- 
tween their anterior and posterior portions. Each of them may be regarded as con- 
sisting of two cylinders, not disjoined in the middle, where the bony portion of the tooth 
is continuous on the crown, but partially separated by a fold of enamel on either side, 
producing a corresponding notch, of which the innermost is the deepest. Placed ob- 
liquely with respect to the general direction of the jaw, they resemble, in some measure, 
a figure of 8 with its elements flattened obliquely, pressed towards each other, and not 
connected together by the transverse middle bars. With the lower molars of Octodon 
those of Poephagomys, as figured by M. F. Cuvier, agree in structure in both jaws. 
Octodon thus evidently exhibits, in its dissimilar molars, the types of two genera: 
the molars of its upper jaw represent those of both jaws of Ctenomys ; those of its lower 
jaw correspond with the molars of both jaws of Poephagomys. 
In the absence from any collection to which I have at present access, of a specimen 
