J. W. DAVIS ON THE GENUS PLEIJTIACANTHTJS, AGASS. 3il 



23. On the Genus Pleura canthtjs, Agass., including the Genera 

 Orthacanthus, Agass. and Gold/., Diplodus, Agass., and Xena- 

 CANTBLUs, Begr. By James W. Davis, Esq., F.S.A., F.G.S., &c., 

 Hon. Sec. Yorkshire Geological & Polytechnic Society. (Eead 

 January 21, 1880.) 



[Plate XII.] 



BiTHEETO specimens of Pleuracanthus have not been discovered in 

 this country which would serve to illustrate the general characters or 

 zoological position of this genus of fossil Pish. Teeth of Diplodus, 

 almost invariably associated with the spines named Pleuracanthus, Ag. 

 and Orthacanthus, Ag., are found in all the great coal-fields. In 

 Staffordshire, Scotland, Lancashire, and Yorkshire, wherever fish- 

 remaius are found, there is some proportion of specimens of these 

 genera. All the examples recorded are from the Coal-measures. 



Though in England we have only the teeth and spines fossil, in 

 Germany and Bohemia several examples have been discovered in 

 which the whole of the fish is preserved. These specimens appear 

 to be higher in the geological series, and have been relegated to 

 certain passage-beds between the Coal-measures and the Permian, 

 and to the Permian rocks themselves. 



The ichthyodorulite Pleuracanthus Icevissimus was described by 

 the late Prof. Agassiz, in his ' Recherches sur les Poissons Possiles,' 

 from an imperfect specimen obtained from the coal-shales of Dudley. 

 On page 330 of the same classical work, whilst discussing the 

 " defenses des Eaies," a second spine is mentioned as somewhat re- 

 sembling Pleuracanthus, and, in aU probability, related to it. It 

 was named Orthacanthus cylindricus, and is figured in the third 

 volume, xDlate 45. figs. 7-9 ; but the description was deferred to a 

 supplementary volume, which, unfortunately, has never been written. 

 At the same time that these spines were discovered, there were also 

 found a number of fossil teeth, which were described and figured by 

 Prof. Agassiz as Diplodus gibbosus. 



In 1847, Goldfuss* described a very fine specimen showing the 

 form of the head, vertebral column, pectoral and ventral fins, and 

 the spines still in position, imbedded in a cartilaginous mass im- 

 mediately behind the occipital region of the head. The spine in 

 this instance is round, has a median ridge on the dorsal aspect ; 

 and on each side of the ridge, separated by a narrow groove, is a 

 row of denticles. This is clearly the Orthacanthus of Agassiz. The 

 fish was named by Dr. Goldfuss Orthacanthus Dechenii. 



In the following year, M. Beyrichf described and discussed the 

 relationship of a fish resembling in aU essential respects the one 

 described by Goldfuss, except that the spine, instead of being round, 

 like Orthacanthus, " is flattened before and behind, and has on each 



* Beitrage zur Eauna des Steintohlengebirges : Bonn, 1847. 

 t Bericht derkonigl. preussischen Akademie derWissenschaften, 1848, p. 24^ 

 Q. J.G.S. No. 143. z 



