240 



DIPNOI 



jaw muscles. Now this ' squamosal ' is generally likened to that of 

 the Amphibia, to which it bears a remarkable resemblance. Since, 

 in Ceratodus sturii (Teller [433]), it harbours the postorbital sensory 

 canal, it is probably the homologue of the pterotic (squamosal) of 

 the hyostylic fishes. As it spreads downwards over the quadrate 

 it appears to be a prespiracular bone, and therefore can hardly 

 represent, as has been suggested, the preopercular of other Osteich- 

 thyes. For, in these, the hyomandibular sensory canal, supplied by 

 the post-spiracular hyomandibular branch of the seventh nerve, is 

 lodged in the preopercular. In fact, a true preoper.cular has ne\;er 



f. 



..-sc. 



Is. 



FIG. i'OO. 



Restoration of the head of Ctenodits. m, maxilla ; pm, premaxilla ; p, median plate ; other 

 letters as in Fi^. 208. 



been shown to exist in the Dipnoi. The two opercular bones are 

 small, but retain on their inner surface small cartilages, which 

 appear to be remnants of branchial rays of the hyoid arch. Bran- 

 chial rays, however, are not found on the gill-arches of modern 

 Dipnoi (Fig. 97). 



The Dipneumones differ from Ceratodus chiefly in the further 

 modification of the dermal bones. The postorbitals and suborbitals* 

 are gone. The * ethmoid ' remains in front, and a large median 

 bone lying on the chondrocranium, and partly below the muscles, 

 probably represents the occipital. The 'laterals' project freely 

 behind over the muscles (Fig. 211). It is obvious that the cranial 

 bones of the modern Dipnoi are in a very specialised condition. 



