THE CROCODILIA. 219 



6. There are larger alisphenoids, but the orbitosphenoids 

 are absent or radimentary. 



7. There is no parietal foramen. 



8. The quadrate bone is very large, and fixed immovably 

 to the walls of the skull, as in the Chelonia; and, as in the 

 latter, the pterygoid bone is firmly connected with the base 

 of the skull, and united only with the upper and inner surface 

 of the quadrate bone. 



9. The pterygoid sends down a large free process, against 

 the broad outer edge of which the inner surface of the mandi- 

 ble plays. 



10. The tympanic cavity is completel}' bounded by bone. 

 The proOtic and opisthotic (which is united with the ex-oc- 

 cipital) form its inner walls, the quadrate its outer wall, the 

 squamosal and post-frontal its roof, and the quadrate, the basi- 

 ocoipital, and basisphenoid its floor. The two tympana are 

 placed in communication with the cavity of the mouth by three 

 canals — one large, opening in the middle line ; and two smaller 

 ones at the sides, on the base of the skull, behind the posterior 

 nares. The large canal passes up between the basisphenoid 

 and basioccipital, and divides between those bones into a 

 right and left lateral canal. Each lateral canal subdivides 

 into an anterior branch, which traverses the basisphenoid, and 

 a posterior, which passes up in the basi-occipital. The 

 posterior branch receives the narrow lateral canal of its side 

 (which runs vertically up to it), and then opens into the 

 posterior part of the floor of the tympanum. The anterior 

 branch opens into its anterior wall. 



The tympanic cavities of embryonic Crocodiles communi- 

 cate with the mouth by wide and simple apertures, and the 

 complicated arrangement of canals just described results from 

 the great downward development of the basisphenoid and basi- 

 occipital, and their encroachment upon these apertures on the 

 inner side, while the quadrate bone narrows them on the outer. 



In adult Grocodilia, air-passages extend from each tym- 

 panum to that of the opposite side, through the bones which 

 form the roof of the posterior region of the skull. On the 

 other hand, they excavate the quadrate bone, whence the 

 air passes through a membranous tube into the hollow ar- 

 ticular piece of the mandible. The hyoidean apparatus is 

 greatly simplified, consisting only of a broad plate of cartilage, 

 which may become partially ossified, anJof two ossified 

 cornua which are not directly conn^r* a with the skull. A 

 minute styliform cartilage, which livs in close proximity with 



