256 THE VERTEBRATE SKELETON. 



elongated slightly convex surface, coated with cartilage in the 

 fresh skull, by which the mandible articulates with the cranium. 

 The dorsal surface of the quadrate is flat behind, further for- 

 wards it becomes much roughened and articulates with the 

 exoccipital and squamosal ; further forwards still it becomes 

 marked by a deep groove which forms "the floor of the ex- 

 ternal auditory meatus and part of the tympanic cavity. 

 The anterior boundary of the quadrate is extremely irregu- 

 lar, it is united dorsally with the postfrontal, pro-otic, and 

 squamosal, and more ventrally with the alisphenoid. The 

 smooth ventral surface looks into the pterygoid fossa. In 

 front the quadrate forms the posterior boundary of the supra- 

 temporal fossa and foramen ovale, and is continuous with the 

 alisphenoid, while it sends down a thin plate meeting the 

 pterygoid and basisphenoid. On the inner side of the dorsal 

 surface of the quadrate near the condyle, is a small foramen 

 which leads into a tube communicating with the tympanic 

 cavity, by a foramen lying in front of and ventral to that for 

 the exit of the facial nerve. By this tube air can pass from 

 the tympanic cavity into the articular bone of the mandible. 



The squamosal (fig. 44, 7) meets the quadrate and exoccipital 

 below, and forms part of the roof of the external auditory 

 meatus, while above it forms part of the roof of the skull and 

 has a pitted structure like that of the other bones of the roof. 

 It is continuous with the postfrontal in front, forming with 

 it the supratemporal arcade which constitutes the outer 

 boundary of the supratemporal fossa. It meets also the 

 parietal on its inner side, forming the post-temporal bar, the 

 posterior boundary of the supratemporal fossa. 



It may be useful to recapitulate the large vacuities in the 

 surface of the Crocodile's cranium. 



Dorsal surface. 



1. The Supratemporal fossae. Each is bounded in- 

 ternally by the parietal, behind by the post-temporal bar 

 formed by the parietal and squamosal, and externally by the 



