510 DIPNOI 



orbitosphenoids are also present (Fig. 250). Of the bony plates 

 derived from fused teeth there are two pairs which represent the 

 vomers, and palatine bones are situated on the roof of the mouth. 

 In the lower jaw there is a membrane bone, the dentary, which 

 carries on each side another dental plate. But the roofing bones of 

 the skull consist of two median bones one behind the other, flanked 

 by a lateral bone on each side, and it appears to be mainly on this 

 ground as also on the absence of premaxilla and maxilla that rela- 

 tionship to the Amphibia is denied, because in Amphibia the roofing 

 bones form a paired series, as in most Osteichthyes, and premaxilla 



...-10 



FIG. 250. Dorsal (to the left) and ventral (to the right) views of the cranium of 

 Ceratodus miolepis. After Giinther. 



1. Cartilaginous part of the quadrate with which the mandible articulates. 

 2, 3, 4. Hoofing membrane bones. 5. Nares. 6. Orbit. 7. Pre- 

 opercular (squamosal). 8. Second rib. 9. First rib. 10. Vomerine 

 dental plate. 11. Palatine dental plate. 12. Pterygoid. 13. Para- 

 sphenoid. 14. Interopercular. 



and maxilla are present and bear teeth, and the upper jaw, though 

 fused with the cranium before and behind, and not movable on it, 

 is not completely fused with it. But of course the most ardent 

 supporter of the view that Dipnoi are related to the ancestors of 

 Amphibia would admit that it is a very long time since the two 

 groups were separated from one another, and that modem Dipnoi 

 have undergone changes in the meantime. As a matter of fact, 

 when we examine fish which are regarded as fossil Dipnoi, chiefly 

 on account of the fusion of their teeth into plates and the shape of 

 their fins, we find that premaxilla and maxilla are present, and 

 that the head is covered with numerous roofing bones, the most 



