Zoological Society. 



507 



todontidce) , which is peculiar to the southern parts of South America, 

 a family of which six species are characterized, and these constitute 

 the four genera, Ctenomys, Poephagomys., Octodon, and Abrocoma. 



The present Rodent agrees with the two first of these genera in 

 having the fore-feet strong and furnished with large claws formed 

 for burrowing, and approaches most nearly, as regards its external 

 characters, to the genus Poephagomys of F. Cuvier, (which appears to 

 be the Psammoryctes of Poeppig,) inasmuch as its ears are of mode- 

 rate size, whilst those of the species of Ctenomys are very small. 



The skull presents a very large antorbital opening, enclosed ex- 

 ternally by the zygomatic process of the superior maxillary bone, 

 the root of which is thrown out almost horizontally, and is on the 

 same plane as the anterior palatal portion of the cranium : a second, 

 very small opening, for the transmission of the infra- orbital nerve, is 

 pierced through the root of the zygomatic process* ; the zygoma is 

 deep and compressed ; the portion of the palate situated between the 

 molar teeth is contracted in front, and widest between the posterior 

 molars ; and the posterior portion of the palate presents a deep V- 

 formed emargination. The descending ramus of the lower jaw, or 

 that part which lies below and behind the alveolar portion, is thrown 

 out from the outer side of the alveolus of the great inferior incisor, 

 which extends almost to the condyle ; its posterior portion is emar- 

 ginated, and lower and hinder portion is produced in the form of an 

 acute angle, terminating behind the vertical line, dropped from the 

 condyle ; the lower boundary of the descending ramus is produced 

 externally and internally in such a manner, as when viewed from 

 beneath, this part presents a horizontal platform of small extent, 

 and which is broadest at about one-sixth of an inch from the angle of 

 the jaw, the platform at this part being produced internally so as to 

 form an obtuse angle. The condyloid portion of the lower jaw has 

 a considerable antero-posterior extent, and the articular surface, 

 which is rather broad, occupies rather more than half of this com- 

 pressed condyloid process. The coronoid process is in the form of 

 an isosceles triangle, the apex of which is of the same height as the 

 condyle, or very nearly so. 



The molar teeth are rootless and four in number on each side of 

 each jaw. 



These characters of skull and dentition all indicate the affinities 

 of the animal under consideration with the Octodontidtf: the cranium 

 compared with that of the known species of the family presents the 

 modifications observable in the burrowing types, differing from the 

 Octodons and Abrocomas, which live more on the surface of the 

 ground and ascend trees and bushes, in the smaller size of the 

 cranial cavity, and in the greater strength of all the parts, arising from 

 the comparatively large size of the teeth, and more especially of the 



* This second small opening is found in the skull of Octodon, and appears 

 to be represented in the figure (given by Mr. Bennett) in Ctenomys, though 

 not so well expressed by the engraver as by the artist of the drawing from 

 which the engraving was made. In Abrocoma there is no corresponding 

 opening. 



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