404 MESSES. HANCOCK AND ATTHEY 



These bones vary a good deal in size and form : some are 

 comparatively narrow and much elongated ; others are short 

 and broad ; but all have the right-angular process at the narrow 

 extremity. The largest are four inches and three-eighths, and 

 the smallest five-eighths of an inch in length. 



These hatchet- shaped bones undoubtedly belong to Ctenodus, 

 as they frequently occur with the remains of that fish ; and a 

 right and left specimen have been found in connexion with a 

 crushed head of C. obliqmis, which fine cranial example exhi- 

 bits three of the dental plates, both opercula, the sphenoid, the 

 occipitals, and several other bones of the head. No jugular 

 plates have been found ; but as they are present in Dipterus, 

 they may be expected to occur in Ctenodus. 



The Hon. William Forster's most interesting discovery of the 

 extraordinary fish which Mr. Gerard Krefft has described under 

 the name of Ceratodus Forsteri, will, no doubt, in due course 

 throw a flood of light on these curious Devonian and Carboni- 

 ferous genera, with which it evidently has much in common. Its 

 relationship to Ceratodus, however, is perhaps doubtful. From 

 Mr. Krefft's description,* it appears that in Ceratodus Forsteri the 

 skeleton is only partially ossified, in this respect agreeing with 

 Dipterus and Ctenodus ; but from what is known of Ceratodus, 

 the latter is probably a true cartilaginous fish, and consequently 

 a Selachian or Placoid. If this were not the case, surely some- 

 thing more would be known of it than the mere dental plates, 

 which do not seem to be uncommon, but which are never found, 

 so far as we know, in connexion with bony supports, with palatal 

 or mandibular bones. In Ctenodus, on the contrary, which has 

 the palatal bones and mandible ossified, the dental plates usually 

 occur attached to them. Mr. Atthey has in his collection nu- 

 merous specimens of the dental plates adhering to entire rami 

 and perfect palato-pterygoid bones. Specimens of all the species, 

 excepting C. corrugatus, occur in this state ; and, in fact, some 

 portion of the bony support is almost always present. The en- 

 tire absence, then, in Ceratodus of any such bony support would 

 seem to indicate that none had ever existed. 



* Proc. Zool. Soc. 1870, part 2, p. 221. 



