122 



THE VERTEBRATE SKELETON. 



Polyodon differs much from Acipenser, the membrane bones 

 not being so well developed though they cover the great carti- 

 laginous snout. 



The skull in Polypterus (Crossopterygii) shows a great 

 advance towards the condition met with in Teleostei. The 

 cranium remains to a great extent unossified, and large dorsal 

 and ventral fontanelles pierce its walls. It is covered by a 



FIG. 18. LATERAL VIEW OF THE SKULL OF A STURGEON (Acipenser 

 sturio). Nearly all the membrane bones have been removed (Brit. 

 Mus.). 



palatine. 



hyomandibular. 



pharyngo-branchial. 



epi-branchial. 



cerato-branchial. 



hypo-branchial. 



nasal cavity. 



orbit. 



parasphenoid. 



vomer. 



pterygoid. 



maxilla. (The dotted line 

 running from 6 passes 

 into the mouth cavity.) 



dentary. 



symplectic. 



10. 

 11. 

 12. 

 13. 

 14. 

 15. 

 16. 

 17. 

 18. 

 19. 



coalesced anterior vertebrae. 



inter-hyal. 



cerato-hyal. 



rib. 



great development of membrane bones, paired nasals, frontals, 

 parietals, supra- and post-temporals, and dermo-supra-occipitals 

 among others being present. The palato-ptery go- quadrate bar 

 is fused to the cranium, and in connection with it the following 

 paired membrane bones appear, palatine, ecto-, meso- and meta- 

 pterygoid, and further forwards jugal, vomer, maxilla and 

 premaxilla. The membrane bones developed in connection 



