129 



in the relative position of the glenoid cavities and zygomatic arches, and 

 in many minor details already alluded to. 



In the aspect of the plane of the occipital foramen and occipital 

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

 in the aspect of the plane of the anterior bony aperture 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 Pachydermata, 

 and manifests an affinity to the Dinotherium and Cetaceous order, espe- 

 cially the Herbivorous section. 



At present we possess no evidence to determine whether the extremi- 

 ties of the Toxodon were organized on the ungulate or unguiculate type, 

 nor can we, be positive, from the characters which the skull affords, that 

 the genus may not be referrible to the Mutica of Linnaeus, although the 

 development of the nasal cavity, and the presence of large frontal sinuses 

 render it extremely improbable that the habits of this species were so 

 strictly aquatic, as the total absence of hinder extremities would occasion. 



Where the dentition of a mammiferous animal is strictly carnivorous, 

 this structure is obviously incompatible with a foot incased in a hoof; 

 but where the teeth are adapted for triturating vegetable substances the 

 case is different. If animals so characterized are of small size, and seek 

 their food in trees, or if they burrow for roots or for shelter, the vege- 

 table type of dentition must co-exist with unguiculate extremities, as in 

 the Edentata and Rodentia generally ; hut the largest genus {Hydro- 

 chcerus) of the Rodent order, whose affinity to the Pachydermata is 

 manifested in its heavy shapeless trunk, thinly scattered bristly hair, and 

 many other particulars, has each of its toes inclosed in a miniature hoof. 



The affinity above alluded to is too obvious to have escaped popular 

 notice, and the Capybara from its aquatic habits has obtained the name 

 of Water-hog. It is highly interesting to find that the continent to 

 which this existing aberrant form of Rodent is peculiar, should be found 

 to contain the remains of an extinct genus, characterized by a dentition 

 which closely resembles the Rodent type, but manifesting it on a gigantic 

 scale, and tending to complete the chain of affinities which links the 

 Pachydermatous with the Rodent and Cetaceous orders. 



Discovered and presented by Charles Darwin, Esq., F.R.S. 

 s 



