_Polyodon_ differs much from _Acipenser_, the membrane bones not being so well developed though they cover the great cartilaginous snout.
[Illustration FIG. 18. LATERAL VIEW OF THE SKULL OF A STURGEON (_Acipenser sturio_). Nearly all the membrane bones have been removed (Brit. Mus.).
1. nasal cavity. 2. orbit. 3. parasphenoid. 4. vomer. 5. pterygoid. 6. maxilla. (The dotted line running from 6 passes into the mouth cavity.) 7. dentary. 8. symplectic. 10. palatine. 11. hyomandibular. 12. pharyngo-branchial. 13. epi-branchial. 14. cerato-branchial. 15. hypo-branchial. 16. coalesced anterior vertebrae. 17. inter-hyal. 18. cerato-hyal. 19. rib.]
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 great development of membrane bones, paired nasals, frontals, parietals, supra- and post-temporals, and dermo-supra-occipitals among others being present. The palato-pterygo-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 premaxillae. The membrane bones developed in connection with each ramus of the mandible are the dentary, angular, and splenial, in addition to the cartilage bone the articular. Several large opercular bones occur. There are also a pair of large jugular or gular plates, and several large opercular bones.
In Bony Ganoids both cartilage bone and membrane bone is well developed. The pro-otics and exoccipitals are well ossified, but the supra-occipital and pterotics are not. Lateral ethmoids are developed, and there are ossifications in the sphenoidal region which vary in different forms. The place of the cartilaginous palato-pterygo quadrate is taken by a series of bones, the quadrate behind and the palatine, ecto-, meso- and meta-pterygoids in front. In _Lepidosteus_, however, the palatine and pterygoid are membrane bones, as they are in _Polypterus_ and the Frog. Paired maxillae, premaxillae, vomers and a parasphenoid occur forming the upper jaw and roof of the mouth, and a series of membrane bones are found investing the mandible and forming the operculum.