422 , 



J. W. DAVIS ON THE TTSH-KE1TAINS OF 



of the others. In the Magazine of Natural History* Mr. Charles- 

 worth has described the fossil remains of a species of Hybodus, col- 

 lected by Miss Anning at Lyme Regis. In connexion with the 

 spine and teeth there is a bone which is undoubtedly an example of 

 SpJienonchus, and is regarded by Prof. Agassiz in that light ; in the 

 same volume (p. 605) there is a letter from Miss Anning, saying 

 that the hooked tooth (Sphenonchus) is by no means new, but that it 

 has been frequently found at Lyme Regis in connexion with the 

 teeth and spines of Hybodus or the teeth of Acrodus. 



Mr. Charlesworth suggested that the hooked tooth is a dermal 

 appendage or defence, probably situated immediately behind the 

 head. Specimens since discovered prove that this suggestion was 

 correct, and that two or three of these bodies were located on 

 the occipital region of the head of Hybodns. The microscopical 

 structure of the teeth and dermal defences of the Elasmobranch 

 fishes is very similar, one of the principal differences being the 

 large size of the pulp-cavity of the dermal processes as compared 

 with that of the teeth ; and in this respect Prof. Agassiz has shown 

 that there is a great difference between the teeth of Hybodus or 

 Cladodus and JSphenonchus. The base of Sphenonchus is excessively 

 expanded, especially in 8. hamatus from the Lias ; and its fibrous 

 structure without enamel indicates that it was imbedded in the 

 flesh ; it appears in this respect to resemble the dermal defences of 

 JRaia clavata from the Tertiary deposits. 



In addition to the specimens already named, there are a number 

 of the bones of the head, including jaws with teeth, oiSauriclithys ?; 

 these I have handed to Mr. Sollas, who has already in preparation a 

 paper on the same genus, derived from the examination of similar 

 specimens in his possession. 



Large numbers of fragmentary bones and small teeth are found 

 scattered throughout the mass of the bed, but without sufficiently 

 well established characters to enable an account to be given of them. 

 A large operculum, nearly 2J inches in diameter, probably belong- 

 ing to Ceratodus, and several bones with articular extremities, 

 which may belong to the same genus, are included in the collection 

 of Mr. Ord. 



Besides the palatal teeth of Psavnmodus and Cochliodus, in all 

 probability derived from the Mountain Limestone which underlies 

 the Rhastic beds in some parts of the area, the following species of 

 Ctenoptycliius have probably been derived from the Coal-measures, 

 and the Petcdodus from within the Coal-measures or the Limestone. 

 Specimens of Helodus have also been met with ; they appear to be- 

 long or are very nearly related to H. simplex. 



Ctenoptychius Ordii, Davis. (Plate XXII. fig. 8.) 



Tooth. Length -55 inch. Depth -3 inch. A portion of the base 

 is wanting. 



The superior surface is folded so as to form a pectinated edge 



* Vol. hi. p. 242, 1839. 



