SKELETON. 



163 



usually occur. These may extend back to the angle of the jaw, 

 or a jugal (malar) and a quadratojugal may intervene, the lat- 

 ter connecting with the quadrate, and in some cases arising in 

 part from an ossification of a process of the quadrate cartilage. 

 In the roof of the mouth in front are usually a pair of vomers, 

 and behind these, and extending back usually to meet the ptery- 

 goids, are a pair of palatines ; while in some groups an os trans- 



Pmx Vo 



0/10 



FIG. 172. Skull of Cydodus from the side and split through the middle, from 

 Huxley. Ar, articulare ; BO, basioccipital ; BS, basisphenoid ; Co, columella; D, 

 dentary; EO, exoccipital; EpO, epiotic; Fr, frontal; Ju, jugal; MX, maxillary; 

 Na, nasal; Op O, opisthotic ; /ft, parietal ; Pf, postfrontal ; PI, palatine; Pmx, pre- 

 maxillary ; Prf, prefrontal ; PrO, prootic ; Pt, pterygoid ; Qu, quadrate ; SO, supra- 

 occipital ; Sy, squamosal ; Vo, vomer ; V, VII, exits of fifth and seventh nerves. 



versum occurs, connecting the hinder portion of the mandible 

 with the pterygoid. In the ichthyopsida the floor of the chon- 

 drocranium does not ossify ; and here the remainder of the roof 

 of the mouth is formed by an unpaired membrane bone, the 

 parasphenoid. 



In the ganoids and all higher forms membrane bones form 

 around Meckel's cartilage, and these form the functional lower 

 jaw. In their greatest development there may be several of 



