THE SKULLS OF REPTILIA AND AVES. 235 



Iii the GrocodiUa the tympanic cavities and Eustachian 

 passages arc still more remarkably disposed.* 



The tympanic cavity of Crocodilus bvporcatus (Fig. 89, A ; 

 Fig. 96) is distinguishable into an inner and an outer part. 

 The latter is bounded by the squamosal bone, above and behind, 

 by the quadrate bone, below and in front. Into the former the 

 supra-occipital enters, above; the quadrate, and, to a slight 

 extent, the basi-occipital and basi-sphenoid below. To the 

 posterior wall of the inner division, that outward and backward 

 prolongation of the ex-occipital, which answers to the opisthotic 

 of the turtle, contributes, while the front wall is formed partly 

 by the quadrate and partly by the pro-otic bones. 



Externally the tympanum opens by the external auditory 

 meatus — its internal wall is formed chiefly by the pro-otic, 

 opisthotic, and epiotic. The two latter are, as I have already 

 stated, anchylosed, respectively, with the ex-occipital and the 

 supra-occipital. 



Each tympanic cavity communicates with its fellow of the 

 opposite side, superiorly, by a wide passage, which perforates 

 the supra-occipital bone and has a secondary diverticulum 

 traversing the ex-occipital. Below, the tympana communicate 

 with one another indirectly, by means of the common median 

 Eustachian tube, the aperture of which, formed, half by the basi- 

 sphenoid and half by the basi-occipital, is seen on the base of the 

 skull behind the posterior nares (c). Each tympanum communi- 

 cates with the common Eustachian tube by two passages : one, 

 wide, from the posterior and inferior part of the tympanum (b) ; 

 and one, very narrow, from its anterior and inferior region (/). 



The two exits are separated by that part of the floor of the 

 tympanum which is formed by the basi-sphenoid and basi- 

 occipital. This presents,, behind, a hemispherical fossa (CI) into 

 which both the basi-sphenoid and basi-occipital enter, and, in 

 front, a round aperture with raised edges, situated altogether 

 in the basi-sphenoid (C a ). The fossa lodges the distal blind end 

 of the cochlea. The aperture leads into a canal, which, passing 



* These were first carefully described by Professor Owen in a memoir published 

 in the Philosophical Transactions for 1850. Windischmann, " De Penitiori auris in 

 Amphibiis Structural " (1831,, has given but a very imperfect account of them. 



