I, — Pleuracanthidce. 743 



cavity, circular in form, extends upwards from the base. The walls of the cavity- 

 forming the base of the spine are thin, and in the specimen now described, they 

 are crushed. 



Mr. A. Smith Woodward* doubtfully places Pleuracanthus liserialis as a synonym 

 of Pleuracanthus cylindricus, Ag. There is, however, a great difference between the 

 two spines. The greatest discrepancy is in the form of the denticles ranged on 

 either side of the dorsal aspect of the spine. In Pleuracanthus ( Orthacanthus) cylin- 

 dricus, Ag. ; the teeth are more or less hooked, pointed, and close together ; but, 

 in this instance, the denticles are widely separated, rounded, and blunt ; to such an 

 extent is this the case that the term is scarcely applicable ; they have more of the 

 character of wavy projections alternately produced ; first right, then left, from the 

 dorsal surface of the spine ; and it was in consideration of this peculiarity that the 

 name was chosen. Even if the spine were worn or abraded, which does not appear 

 to be the case, the great difference in the number of the denticles must distinguish 

 it from P. cylindricus, Ag., which has six or seven times as many denticles as the 

 spine now described. 



In the original descriptiont of the specimens forming this species attention was 

 drawn to their resemblance to Orthacanthus, the principal points of difference 

 being in the curved contour of the spine, and in the peculiar form and large 

 size of the posterior denticles. During the following year a number of additional 

 specimens were discovered forming intermediate stages between Pleuracanthus and 

 Orthacanthus which led to a suggestion that the two genera should be combined 

 along with Xenacanthus and Diplodus,X and all form only one genus. After con- 

 siderable care in comparing specimens, it appears probable that Phricacanthus must be 

 included in the genus Pleuracanthus. The latter now includes not only the straight 

 spines of Orthacanthus type, but also the curved spines, since allocated to the genus, 

 and so covers one of the characteristic features of Phricacanthus ; and the denticles 

 which are now known to be extremely varied in that genus may well embrace the 

 double row of large, widely separated and alternate denticles of Phricacanthus. 

 The specific name biserialis is pre-occupied, having been applied to a species of 

 Pleuracanthus from the Coal Measures of Ohio by Dr. J. S. Newberry§ in 1856. 

 It is now proposed to distinguish the species as Pleuracanthus obtusus, Davis. 



Formation and locality. — Bone-bed above Better-bed Coal ; Clifton, Yorkshire. 



Ux coll. — James W. Davis, Halifax. 



* Cat. Foss. Fishes, Brit. Museum, p. 8. 1889. 



f Quart. Joum. Gaol. Soc, vol. xxxv., p. 186. 1879. 



J Op. cit., vol. xxxvi., p. 325. 1880. 



§ Proc. Acad. Nat. Sci., PhUadelpMa, p. 100. 1856. 



TBANS. EOY. DTTB. SOC, N.S. VOL. IV., PAST XIV. 5 Q 



