189 



December 22, 1839^ 

 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 

 Herbivorous Rodentia, Octodon, Benn., and Poephagomys, F. Cuv., 

 a specimen of Octodon Cumingii was exhibited and compared ■vvith 

 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 upi)er 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 out\vardly 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 

 \vith the structure that exists in the upper jaw of Octodon, excepting 

 that their crowTis are slenderer and more obliąuely placed, whence 

 the extemal 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 cro\vn of the tooth, \vhich in the upper jaw is extemal, 

 is, in the lower ja\v, internal. In the lower ja\v of Octodon the crowns 

 of the molars assume a figure very differeiit from thosc of the upper, 

 dcpendcnt chicfly on the prolongation of the hinder portion of the 

 tooth to the samc latcral extcnt a? its anterior jiart : each of them 



