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tamus, but the size of the orbit is relatively larger in the Toxodon, which 

 in this respect manifests its affinity to the Rodentia. 



In that part of the bony structure of the auditory apparatus which is 

 visible on the exterior of the cranium, the skull of the Toxodon presents 

 a character in which it recedes from the Rodentia. In these quadrupeds 

 the tympanic portion of the temporal bone is remarkably developed, 

 forming a large bulla ossea between the glenoid cavity and the occiput ; 

 and it always remains disunited from the other elements of the temporal 

 bone, in the Toxodon the tympanic bone consists of a rough, com- 

 pressed, vertical, osseous plate, wedged in transversely between the occi- 

 put and the posterior part of the glenoid cavity. The internal extremity 

 of this plate points inwards and forwards, representing the styloid pro- 

 cess ; behind this is seen the petrous boue, which forms a small angular 

 protuberance at the basis cranii, and is less developed than in the Hippo- 

 potamus. Anterior to the petrous bone are the orifices of the Eusta- 

 chian tube and carotid canal, external to it is the great foramen lacerum 

 for the jugular vein and nervus vagus, and behind it is the anterior con- 

 dvloid foramen. The foramen auditorium externum is only half an 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 indication of the aquatic habits of the Toxodon, which are 

 presented by the osseous parts relating to the senses of sight and hearing, 

 is of minor import compared with those afforded by the bony boundary 

 of the nostrils. This boundary circumscribes a large ovate aperture, the 

 aspect of whose plane is upwards, and a little forwards, as in the Herbi- 

 vorous Cetaceans and especially the Manatee [Trichecas Manatus, Cm'.). 

 In one part of the bony structure of the nasal cavity the Toxodon devi- 

 ates, however, in a marked degree from the cetaceous organisation, viz. 

 in the presence of frontal air-sinuses, which are exposed by the fracture 

 of the upper part of the skull. 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 



