FOSSIL REPTILES. 215 



The OS tympani exhibits a large concave surface underneath, 

 which articulates with the sphenoid, the pterygoid, and the 

 great ala temporalis. Between this last and the os tympani is 

 pierced the foramen, through which the fifth pair passes. This 

 is the same with many mammalia in relation to the foramen 

 ovale. 



The hinder free edge of the os tympani^ which projects be- 

 hind, supports almost entirely the articulary facet for the 

 lower jaw. This is nothing very anomalous, for in several 

 mammalia, and even in man, the bone of the tympanic closet 

 begins to contribute to form the posterior edge of the articular 

 cavity. 



All these functions of the tympanic bone in the crocodiles 

 are performed in birds by the bone which has been named os 

 quadraturriy and the latter is distinguished from the former 

 only by its mobility. And we may consider the os quadratum 

 a true tympanic bone. 



This bone, however, in the crocodile, does not include the 

 whole bony cavity of the tympanum, even abstracting the mas- 

 toid cellulae, but neither does it do so in the mammalia. The 

 os petrosum, the temporal squamous process, and frequently 

 the sphenoid, contribute also to the formation of the same 

 part. This grand cavity is formed of many bones. Its internal 

 paries is always the os petrosum. The os tympani forms, in 

 general, a great portion of this cavity, constituting the entire of 

 its external and lower paries. 



The squamous temporal piece is entirely separated from the 

 cranium, an approach to which arrangement we find already 

 commencing in the ruminantia and cetacea. 



The mastoid piece of the crocodiles proper and gavials has 

 this peculiarity, that it advances laterally to unite itself with 

 the posterior frontal, and to surround with it, and the parietal, 

 the foramen of the upper face of the cranium which communi- 

 cates with the fossa temporalis. In some caymans it even unites 

 with these three bones to cover this fossa altogether above. 



