J. W. DAVIS ON THE GENUS PIEIJEACANIHTJS, AG ASS. 329 



P. aUernidentatus is more nearly related to P. alatus than to any- 

 other. It is, however, easily distinguished by its more robust ap- 

 pearance, the diameter being greatest between the anterior and pos- 

 terior surfaces, whilst in P. alatus the transverse diameter is greater. 

 The walls of the spine in this species are much thicker and stronger 

 than in P. alatus. The point of most divergence consists perhaps in 

 the widely separated and alternate denticles. 



6, Pleueacanthus planus, Agass. (sp. indet.). 



In the 'Poissons Fossiles,' tome iii. p. 176, M. Agassiz records a 

 spine of Pleura canthus, to which he has appended the specific name 

 planus ; it is said to be from the Coal at Leeds. Sir Philip Egerton, 

 who possesses the type of P. planus, writes me " that it is about J 

 an inch in length, the basal end being absent ; there are six or seven 

 strong booklets on each side imbedded in the matrix. The exposed 

 surface is quite smooth and flat." 



7. Pletjracanthtjs alatus, sp. nov. PI. XII. fig. 4. 



Spine. Length 2*2 inches, breadth '2 inch ; the general form is 

 straight. A slight appearance of curvature is given by the anterior 

 face curving from the base to the apex, whilst the posterior is 

 straight. From the basal end to the middle the spine has been 

 crushed ; it appears to have been uniform in diameter ; from mid- 

 length to the apex it becomes gradually smaller, and ends in a fine 

 point. It is uniformly striated on the anterior and posterior sur- 

 faces ; the intermediate furrows are frequently broken into a suc- 

 cession of pittings, especially the lower parts. There is an internal 



Pig. 6. — PleuracantJius alatus, Davis. 



a. Section of spine, nat. size. b. Denticles, much enlarged. 



-cavity, open and large at the base. In section the posterior face 

 forms a much-depressed curve, the anterior curvature forming a semi- 

 circle above it (fig. 6 a) ; the lateral angles formed by the junction 

 of the two are ornamented or armed with a row of denticles (fig. 6 b), 

 numbering ten on each side. They extend from the point -8 of an 

 inch. The denticles are broad at the base, closely set and short, 

 terminating obtusely, with a slightly trenchant edge parallel to their 

 longitudinal axis. Those nearest the apex of the spine are less pro- 

 duced than those lower down. 



Locality. Cannel Coal, Tingley (M. CM.). 



