THE SKULLS OF MAMMALIA. 255 



The tympanic element is very singularly formed. It lias 

 the shape of a very irregular hoop, open above and behind, and 

 much thicker at its anterior superior than at its posterior su- 

 perior end. The former, irregular and prismatic, is anchylosed 

 with the periotic, just behind and above the auditory labyrinth ; 

 it then splits into two divisions, an anterior and inner and a 

 posterior and outer. The anterior, acquiring a thick and 

 spongy texture, curves round to form the front part of the wall 

 of the tympanum, and then ends in a free, backwardly-directed 

 apex, without becoming in any way connected with the periotic, 

 or with the posterior division. The latter, much thinner and 

 denser, curves downwards and backwards in the same way, and 

 also remains perfectly free, but its hinder end is prolonged into 

 a flat process, which bends for a short way round the base of 

 the styloid process. The outer wall of the tympanum is there- 

 fore very incomplete in the dry skull, opening forwards and 

 downwards, first, by the fissure between the anterior branch of 

 the tympanic and the periotic ; and, secondly, by the cleft 

 between the two divisions of the tympanic. 



Posteriorly, there is a large irregular aperture between the 

 hinder end of the anterior branch of the tympanic and the peri- 

 otic. Externally, there is no bony auditory meatus — or rather 

 the merest rudiment of one. 



The Horse presents a very different structure. There is a 

 tympano-periotic bone which is wedged in between the squa- 

 mosal and adjacent bones, and not anchylosed therewith ; but 

 the pars mastoidea appears largely on the outside of the skull 

 between the post-auditory process of the squamosal and the 

 paramastoid, and the tympanic element consists of a complete 

 hulla, with a long external auditory meatus. The styloid pro- 

 cess is almost completely infolded by a vaginal process furnished 

 by the auditory meatus, and the tympanic is altogether anchy- 

 losed to the periotic, posteriorly. 



No Tapir's skull which I have examined has presented any 

 trace of an ossified tympanic bone.* 



In the Horse, most Primates, Carnivores and Rodents, the 



* According to Cuvier, " L'os de la caisse ne paroit jamais bien se souder avec 

 les os voisins et tombc aisement, comme dans l'herisson, le sarigue," &c. 



