VII. On the Genus Octodon, and on its Relations with Ctenomys, Blainv., and Poe- 
phagomys, F. Cuv.: including a Description of a New Species of Ctenomys. By 
E. T. Bennett, Esq., F.L.S., Sec. Z.S. 
Communicated December 22, 1835. 
WHEN early in 1832 I laid before the Society a specimen of a previously unde- 
scribed Herbivorous Rodent brought by Mr. Cuming from South America, and pointed 
out the characters by which it was generically distinguished from all the groups that 
were at that time known, I stated it to be my intention to defer giving a more formal 
account of it until there should occur, by the death of one of the individuals which were 
then living in the Menagerie, an opportunity of entering into some details respecting 
its anatomy, both visceral and osteological. That opportunity has not yet arrived ; and 
there are, consequently, at my disposal no other materials than those of which I 
availed myself when characterizing the animal in the ‘ Proceedings of the Committee 
of Science and Correspondence’!, as the type of a new genus, under the name of Oc- 
todon Cumingii. To these, however, I am induced again to call the attention of zoolo- 
gists, at an earlier period than I had originally proposed, with the view of elucidating 
the relations of Octodon with the nearly allied genus Ctenomys, described by M. de 
Blainville in the ‘ Bulletin de la Société Philomathique’ for April, 1826,? and of which 
a hitherto undescribed species is contained among the collections made by our excellent 
colleague, Capt. P. P. King, R.N., during his survey of the Straits of Magalhaens ; as 
well as with another form which is even more closely connected with it, and which was 
first made known to science by M. F. Cuvier, in the ‘ Annales des Sciences Naturelles’ 
for June, 1834°: to the latter M. F. Cuvier has given the name of Poephagomys, the 
species on which it is founded being the Poeph. ater. The affinity between Ctenomys, 
Octodon, and Poephagomys was first indicated by M. F. Cuvier, who, in a letter ad- 
dressed to me in September last (some extracts from which were immediately commu- 
nicated to the Society’), made known to me the fact that the molar teeth in Ctenomys 
are destitute of true roots. 
In the little group which these three genera appear to constitute, the genus Octodon 
may be regarded as occupying a central station ; if, as is generally admitted, the struc- 
ture and form of the molar teeth be considered as of primary importance in the arrange- 
' Part ii. p. 46. $'p. 62. 
3 Seconde Serie. Zoologie. Tome i. p. 321. * Proceedings Zool. Soc., part iii. p. 128. 
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