214 FOSSIL REPTILES. 



the OS petrosum, the upper, and lateral occipitals, concur. 

 The upper and lower serai-circular canals wind in narrow 

 tubes hollowed in these same parietes, and, consequently, in 

 these three bones. 



The portion of these parietes which separates the vestibulum 

 from the cavity of the cranium is very slender, and divided by 

 a suture with three branches, which marks the limits of the 

 three bones. 



On the side of the os tympani the paries is pierced by two 

 transversely-oblong fenestrse, separated by a thin division. The 

 upper one, which answers to the fenestra ovalis in man, and 

 which is closed by the osselet of hearing, is formed partly by 

 the OS petrosiim, and partly by the lateral occipital. And the 

 other, which is analogous to the rotunda, is altogether in the 

 lateral occipital to which the separating division belongs. 



These two fenestrse are elongated from front to back. They 

 open into the same osseous cavity, which is pretty large ; but 

 a slender ridge, proceeding from the bottom and anterior par- 

 tition of this cavity, and continued in the fresh subject by a 

 membrane, divides it into two parts, of which that which is 

 lower and anterior, and communicates with the under fenestra 

 (^rotunda in man), contains a small lenticular mass, of a sub- 

 stance resembling hardened starch, and quite analogous to 

 what is found in the sac of the ear of thornbacks and squali. 

 This external and anterior part evidently represents the coch- 

 lea. But it is far from being so much developed even as it is 

 in birds, in which it is yet considerably less than in the mam- 

 malia from its trifling inflexion. Still in the aves, especially 

 in the ulula, there is found in it a demi-osseous partition, sen- 

 sibly tending to a spiral curve. The internal and upper part, 

 into which the upper fenestra (ovaJh) opens, as do also the 

 semi-circular canals, is the vestibulum. 



This extension of the two cavities of the auditory organ, in 

 the different bones, is found more or less in all the ovipara. 

 That of the tympanic cellulae, in particular, is much larger in 

 certain birds. 



