FOSSIL MAMMALIA. 109 



became partially resolved into its component curved lamellae. Every point of 

 comparison that could be established proved it to differ from the tooth of the 

 common Equus Caballus only in a slight inferiority of size. 



The second evidence of the co-existence of the horse with the extinct Mam- 

 mals of the tertiary epoch of South America reposes on a more perfect tooth, 

 likewise of the upper jaw, from the red argillaceous earth of the Pampas at Ba- 

 jada de Santa Fe, in the Province of Entre Rios.* 



This tooth is figured at PI. XXXII. fig. 13 and 14, from which the anatomist 

 can judge of its close correspondence with a middle molar of the left side of the 

 upper jaw. 



This tooth agreed so closely in colour and condition with the remains of the 

 Mastodon and Toxodon, from the same locality, that I have no doubt respecting 

 the contemporaneous existence of the individual horse, of which it once formed part. 



This evidence of the former existence of a genus, which, as regards South 

 America, had become extinct, and has a second time been introduced into 

 that Continent, is not one of the least interesting fruits of Mr. Darwin's palseon- 

 tological discoveries. 



DESCRIPTION OF REMAINS OF RODENTIA, INCLUDING THE JAWS AND TEETH OF 



AN EXTINCT SPECIES OF 



CTENOMYS. 



The fragment of the upper jaw, figured in PI. XXXII. fig. 6, exhibits the 

 first and second molar in situ, and the socket of the third and fourth molar, of a 

 Rodent, which by the form and number of the upper maxillary teeth is referable 

 to the genus Ctenomys. The molars are a little larger, the longitudinal groove on 

 their external surface is somewhat deeper, and the last molar is relatively wider 

 than in the existing subterraneous species, — the Tucutucu (Ctenomys Brasiliensis, 

 Bl.), of whose habits so interesting an account is given in the description of the 

 Mammalia of the present Collection (No. IV. p. 79). The form of the grinding 

 surface of the first and second upper molar is shown below the fig. 6, and three 

 views of the second grinder are given at figs. 7, 8, and 9. The fragment of the 

 lower jaw of the same fossil Rodent is figured at fig. 10 and 11. The long anterior 

 incisor is relatively narrower than in the Ctenomys Brasiliensis. I have not had 

 the means of comparing this fossil with the Ctenomys Magellanicus ; but since it is 

 probable that the Ct. Magellanicus may not be specifically different from the Ct. 



* Mr. Darwin has more particularly described the circumstances of the embedment of this tooth in his 

 Journal of Researches, p. 149, during the Voyage of the Beagle. 



