296 COMPARATIVE ZOOLOGY OF CHORDATES 



skull certain bones come to form part of the wall of the brain- 

 case although primitively they had nothing to do with it. 

 This applies to the mammalian alisphenoid, and to the 

 squamosal in birds and mammals. 



Primitive forms have a large number of bones on the 

 palatal surface of the skull. The pterygoids, of which the fish 

 have three on each side, become more and more reduced in the 

 higher forms. The transpalatine of amphibia and reptiles 

 corresponds to the ectopterygoid of the fish, and it disappears 

 in birds and mammals. In the Tetrapods the pterygoid is a 

 membrane-bone, underlying the pterygo-quadrate cartilage. 



The articulation of the pterygoid with the basipterygoid 



Pp.* 



Fig. 150. — Side views of the lower jaws of A, Varanus ; B, a dog. 

 The mammalian lower jaw contains only one bone ; the dentary. an, 

 angular ; ar, articular ; c, coronoid ; d, dentary ; ml, first molar ; par, 

 supra-angular. 



process of the basisphenoid, as for instance in Varanus, corre- 

 sponds roughly to the connexion between the pterygo- 

 quadrate cartilage and the brain-case by the basal process. 



In crocodiles, some Theromorph reptiles and in mammals, 

 the maxillae and palatines have shelf-like extensions which 

 meet in the middle line beneath the original roof of the mouth. 

 These shelves are the false palate, and between it and the 

 original roof of the mouth (formed by the vomer and meseth- 

 moid bones) is the nasal passage leading from the external 

 nostrils to the secondary choanae. The prevomers of the 

 lower vertebrates are represented by the " dumb-bell-shaped 



