Chap. IX.J 



SKULL OF PISHES. 



329 



number of investing membrane bones appear on 

 the roof of the skull ; in the Holosteous Ganoids ossifi- 

 cation commences in 

 the occipital region of 

 the cartilaginous cra- 

 nium, while there are 

 also membrane bones. 

 From this point for- 

 ward we have to dis- 

 tinguish between bones 

 that are preformed in 

 cartilage (cartilage 

 bones), and those 

 that are preformed in 

 membrane (mem- 

 brane bones). 



At first, that is, in 

 the lower Vertebrata, 

 the membrane bones 

 are numerous, and 

 their relations are not 

 so constant and exact 

 as they are in the 

 higher forms. When 

 they become so we are 

 able to recognise that 

 the roof is formed by 

 two pairs of more or 

 less large bones, the 

 parietals abutting 

 on the occipital re- 

 gion, and thefrontals 

 in front of the parie- 

 tals. The base of the 



skull is in the Ichthyopsida ossified in the occipital 

 region only, and the sphenoidal portion is under- 

 laid by a membrane bone, the parasphenoid 



Fig. 138. Head of Sturgeon, showing the 

 Membrane Bones, aud the Cartilaginous 

 Cranium, which is shaded dark. (After 

 Gegenbaur.) 



