206 Geological Society, — Mr. Darwin on the Deposits 



sense is adverted to $ and deductions as to the aquatic habits of the 

 Toxodon are founded on these observations. 



So far as regards the form and position of the external aperture of 

 the bony nostrils, and of the occipital condyles, and the slope of the 

 plane of the occipital region of the skull, the same arguments might 

 be advanced for referring the Toxodon to the mammiferous group 

 containing the Dugong, as have been recently urged in reference to 

 the Dinotherium, but the existence of air-cells or sinuses in the su- 

 perior parietes of the cranium in the Toxodon, show that the cranial 

 characters above alluded to, are not conclusive as to the cetaceous 

 nature of an extinct mammal. 



The general conclusions respecting the affinities which the Toxodon 

 bears to existing orders of mammalia, so far as opinions can be 

 formed from the portion of the skeleton preserved, are summed up 

 by the author as follows : 



So far as dental characters have weight, the Toxodon must be re- 

 ferred to the rodent order ; but from this order it deviates in the 

 relative position of the supernumerary incisors, and in the number 

 and direction of the curvature of the molars. 



It again deviates in the transverse direction of the joint of the 

 lower jaw, and in the relative position of the glenoid cavities and 

 zygomatic arches. In the aspect of the plane of the occipital fora- 

 men, and occipital region of the skull, in the form and position of 

 the occipital condyles, — the aspect of the plane of the bony aper- 

 ture of the nostrils, and in the thickness and texture of the osseous 

 parietes of the skull, the Toxodon deviates both from the Rodentia 

 and existing Pachydermatu , and manifests an affinity to the Dino- 

 therium and the Cetaceous order. 



The author observes, however, that the development of the nasal 

 cavity and the presence of frontal sinuses, render it extremely im- 

 probable that the habits of the Toxodon were so exclusively aquatic 

 as would result from the total absence of hinder extremities, and 

 concludes, therefore, that it is a quadruped, and not a Cetacean ; and 

 that it manifests an additional step in the gradation of mammiferous 

 forms leading from the Rodentia, through the Pachydermata to the 

 Cetacea ; a gradation of which the water-hog of South America 

 (HydrochcerusCapybara) already indicates the commencement amongst 

 existing Rodentia, of which order, it is interesting to observe, that 

 this species is the largest, while at the same time it is peculiar to the 

 continent in which the remains of the gigantic Toxodon were dis- 

 covered*. 



May 3. — A paper was first read, entitled "A Sketch of the Depo- 

 sits containing extinct Mammalia in the neighbourhood of the Plata;" 

 by Charles Darwin, Esq., F.G.S. 



Mr. Darwin premised his account of the geological features of the 

 district in which the remains of the Toxodon, described at the meet- 

 ing on the 19th of April by Mr. Owen, as above, were found, by re- 

 marking that as the other mammalia and the fossil shells had not yet 

 been accurately examined, the notice was necessarily imperfect. 

 [* See note *, p. 208.] 



