FOSSIL MAMMALIA. 



23 



inch in diameter, and gives passage to a long and somewhat tortuous meatus, 

 which passes inwards and slightly forwards and downwards; its direction being 

 precisely the same as in the Hippopotamus ; it was accompanied, probably, by as 

 small an external auricle. 



But the indications of the aquatic habits of the Toxodon, which are presented 

 by the osseous parts relating to the senses of sight and hearing, are of minor im- 

 port compared with those afforded by the bony boundary of the nostrils. This 

 boundary circumscribes a large ovate aperture, the aspect of whose plane is up- 

 wards, and a little forwards, as in the Herbivorous Cetaceans, and especially the 

 Manatee (Trichecus Manatus, Cuv.) In one part of the bony structure of the 

 nasal cavity the Toxodon deviates, however, in a marked degree from the Cetaceous 

 structure ; I allude to the frontal sinuses, which are exposed by the fracture of 

 the upper part of the skull. (They are shewn in Plate III., and an asterisk is 

 placed on one of the narrow canals of intercommunication between the sinuses and 

 the nasal passages.) The posterior orifice of the nasal cavity is relatively larger 

 and wider than in the Herbivorous Cetaceans, and differs both in form and aspect 

 in consequence of the greater extent of the bony palate. The Toxodon further 

 differs from the Manatee and Dugong, in the firm nature of the connexion of the 

 bones of the head ; and it differs from the Hippopotamus in the strong attachment 

 of the intermaxillary bones to the maxillaries. 



There next remain to be described, as far as the shattered condition of the 

 skull will permit, the relative position, extent, and connexions of the principal bones 

 composing it. 



The occipital bone exhibits a complete confluence of its basilar, condyloid, and 

 supra-occipital elements. The basilar portion, in connexion with the correspond- 

 ing element of the sphenoid bone, describes a curve whose convexity is down- 

 wards. The condyles are large, extended in the transverse direction, completely 

 terminal, and a little inclined downwards below the level of the basilar process. 

 The curve of the articulating surface describes, in the vertical direction, two- 

 thirds of a circle, indicating that the head must have possessed considerable 

 extent of motion upwards and downwards upon the atlas ; thus, while the 

 body of the Toxodon was submerged, the head probably could be raised so as to 

 form an angle with the neck, and bring the snout to the surface of the water 

 without the necessity of any corresponding inflection of the spine. Indeed, in 

 the form and position of the condyles, the Toxodon more nearly resembles the 

 true Cetacea than any other existing mammalia ; and it is only with these that 

 it can be compared in regard to the aspect of the plane of the occipital foramen, 

 and of the occipital region of the skull. This is inclined forwards from the occipi- 

 tal foramen at such an angle, that on viewing the skull from above, not only the 



