eS 
MR. E. T. BENNETT ON THE GENUS OCTODON. 79 
opening of the auditory passage ; behind which, for the short portion of the mass that 
remains, the cranium shelves away, both on its sides and upper surface, towards the 
comparatively small flattened face surrounding the foramen magnum and opposed to the 
neck. With the exception of the abruptness of this posterior shelving in Octodon, the 
outline of the upper surface of the cranium may be described as constituting in both 
animals a slight and regular curve ; which is, however, more gentle in Ctenomys than in 
the one with which it is now compared. The general outline, viewed from above, is in 
Octodon of an ovate form ; in Ctenomys it resembles that of a lengthened triangle, trun- 
cated at the apex. If, however, the comparison in the vertical view be limited to that 
portion of the bony mass which lies anterior to a line crossing the skull at the upper 
and hinder part of the orbit, the outline in Ctenomys will be found to be nearly paral- 
lelogrammic, while that of Octodon will resemble a truncated and lengthened triangle, 
widening posteriorly: a figure which is produced chiefly by the greater comparative 
breadth of the frontal bones, consequent on the greater width of the ascending ramus 
of the incisive at the point where it is united with the branch of the maxillary forming 
the slender line of bone that separates the orbit from the large suborbital foramen. In 
both these animals, as in most of the tribe, the infra-orbital foramen is single, and 
attains its maximum of possible development ; involving the whole of the external face 
of the maxillary bone except a slender process passing to where the malar joins it after 
limiting the lower edge of the orbit, and of another equally slender process arising from 
this point to form the anterior margin of the orbit and unite above with the frontal and 
the ascending ramus of the incisive bone. Excepting in thus limiting below the infra- 
orbital foramen, and in separating that foramen by a hinder margin from the orbit, the 
maxillary bone is reduced, in animals of this type, to an alveolar ridge for the entire 
series of the molar teeth, and to affording capacious space for the implantation and 
growth of the prolonged roots of the exceedingly developed upper incisor. 
To the subjoined table of some comparative admeasurements of the crania of these 
two animals the remark must be prefixed that the individual of Ctenomys is not yet fully 
adult, as is evidenced by the incomplete state of closure of its anterior fontanelle. 
Octodon. Ctenomys. 
In. In. 
Length ofthe skull] . . . . 2 NE, to tan Shes 1G 1°75 
Breadth of the skull at the meatus auditorsi 3 whe meciniss 2) ycoian SETI: 95 
Breadth of the skull at the zygomata . . . . . . . 9 1- 
Distance between the orbits above. . . . . . . . ‘45 4 
Diastematic distance, upperjaw, . ....... 4 55 
Diastematic distance, lowerjaw, . .. .. ... # °8 “4 
Length of the molar series . . . 33 "33 
Length of the mastoid process or mass = the iets ats “45 6 
