18.0 



b' 

 December 22, 183(?( 



E. S. Hardisty, Esq., in the Chair. 



Specimens were exhibited of several Rodent animals collected du- 

 ring his survey of the Straits of Magalhaens, by Capt. P. P. King, 

 R.N., Corr. Memb. Z. S., and presented by him to the Society. 

 They were accompanied by some notes by Capt. King, which were 

 read. 



In bringing the animals severally under the notice of the Meeting, 

 Mr. Bennett first directed particular attention to one of them, which 

 constituted, in his estimation, a new species in the genus Ctenomys, 

 Blainv. To elucidate its relations with the nearly allied genera of 

 Herbivor-ous Rodentia, Octodon, Benn., and Poej}liacjomys, F. Cuv., 

 a specimen of Octodon Cumingii was exhibited and compared with 

 it; and Mr. Bennett stated his intention of entering with some 

 detail into the subject in a paper which he proposed to prepare 

 upon it. 



In the structure of its molar teeth, Octodon may be regarded as 

 occupying an intermediate station between Poephagomys and Cteno- 

 mys. In Octodon the molars of the upper jaw differ remarkably in 

 form from those of the lower. The upper molars have on their inner 

 side a slight fold of enamel, indicating a groove tending in some 

 measure to separate on this aspect the mass of the 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 tooth of the upper 

 jaw be regarded as composed of two partially united cylinders, 

 slightly compressed from before backwards, and somewhat oblique 

 in their direction, the anterior of these cylinders might be described 

 as 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, all 

 the molar teeth, both of the upper and the lower jaw, correspond 

 with the structure that exists in the upper jaw of Octodon, excepting 

 that their crowns are slenderer and more obliquely placed, whence 

 the external emargination becomes less sharply defined ; and also 

 excepting that the hinder molar in each jaw is so small as to be 

 almost evanescent : as is generally the case, however, the relative 

 position of the teeth is counterchanged, and the deficiency in the 

 outline of the crown of the tooth, which in the upper jaw is external, 

 is, in the lower jaw, internal. In the lower jaw of Octodon the crowns 

 of the molars assume a figure very different from tliosc of the upper, 

 dependent chiefly on the prolongation of the hinder portion of the 

 tooth to the same lateral extent as its anterior jiart : each of them 



