36 



213'. The corresponding tooth of the Casfor Canadensis. College. 



Besides the striking difference of size, the fossil is distinguished by the 



transverse convexity of the outer enamelled surface of the tooth, which 



describes half a circle, and by the concavity of the inner or median 



facet, which is flat, in the Canada Beaver. 



Genus Ctenomys. 



214. A small fragment of the superior maxillary bone, with portions of the four 



sockets, and the first and second molar teeth of the Ctenomys antiquus. 

 The molars are somewhat larger, the longitudinal groove on their ex- 

 ternal surface is somewhat deeper, and the last molar is relatively wider 

 than in the Ctenomys Brasiliensis . 



215. A portion of the left ramus of the lower jaw, with the incisor and first 



molar tooth, of the Ctenomys antiquus. 



216. The left lower incisor tooth of the Ctenomys antiquus. 



The preceding fossils were discovered in a reddish earthy stratum at 

 Monte Hermoso, Bahia Blanca, Patagonia, and were presented to 

 the Royal College of Surgeons by Charles Darwin, Esq., F.R.S. 

 They are described and figured in the Zoology of the Voyage of the 

 Beagle, Fossil Mammalia, p. 109, pi. xxii. figs. 6 — 11. 



Genus Incognitum. 



217. A considerable portion of the skeleton of the right hind-foot of a small 

 Rodent quadruped. It includes the astragalus, calcaneum, cuboides, ex- 

 ternal cuneiforme, middle cuneiforme, and the metatarsals and proximal 

 phalanges corresponding with those of the three middle toes of five-toed 

 quadrupeds : a rudiment of one inner toe is indicated by an articular 

 facet on the tibial side of the expanded base of the second metatarsal. 



This fossil iv as discovered at Monte Hermoso, in the same stratum as 

 the preceding fossils, and ivas presented to the College by Charles 

 Darwin, Esq., F.R.S. It is described and figured in the work 

 above-cited, p. 110, pi, xxii. fig. 12. 



