FOSSIL REPTILES. 213 



which envelopes all the membranous labyrinth. — 4. The mas- 

 toid portion which covers the os petrosum behind the squamous 

 portion and the os tympanic but which is so soon soldered to 

 the OS petrosum, that it can scarcely be recognized as distinct 

 in the youngest foetuses, in which it is sometimes double. 



In the crocodile we find an os tympani, and three other 

 bones, two of which are external to the cranium, and one 

 altogether internal. 



The OS tympani is easily recognized, for it gives an attach- 

 ment to the membrana tympani, forms a lodgment for the 

 osselet of the organ of hearing, and contributes to form, in a 

 great measure, a cavity in front of the two fenestrse, from the 

 bottom of which cavity proceeds the Eustachian tube. 



The OS petrosum is equally observable by its internal posi- 

 tion, and by its lodging in a great measure the labyrinth, and 

 essentially contributing to the formation of one of the fenestras; 

 but, in the crocodile, neither the os tympani, nor the os petro- 

 sum, is sufficient to lodge the cavity of the tympanum and the 

 labyrinth. 



The OS tympani communicates with some large cells analo- 

 gous to the mastoid, or mammillary cells in man, some of 

 which extend into the lateral occipital piece, and others into 

 the upper occipital. These are common to the ossa tympani 

 of both sides, and unite the two cavities. 



The same is the case with the Eustachian tube. It com- 

 mences in a sinking of the bottom of the cavity of the os tym- 

 pani, descends almost vertically, passes between the basilary, 

 the sphenoid, and the lateral occipital, and terminates in the 

 skeleton, at the point in which these three bones unite: but it 

 is afterwards continued by a membranous tube, and approaches 

 to its correspondent part to arrive, by a common aperture, into 

 the hinder mouth, behind the back nostrils. 



The labyrinth, like the tympanic cavity, and the tube, is 

 surrounded by many bones. Its principal part, the vestibu- 

 lum, is lodged in a cavity, to the formation of whose parietes 



