328 J. W. DAYIS ON THE GENUS PLEUKACANTHUS, AGASS. 



Fig. 4. — Pleurcicanthus pidchellus, Davis. 



Denticles of spine, enlarged, 



quently crushed ; the upper two thirds of the length are armed on 

 each lateral face with about twenty exquisitely beautiful little 

 denticles ; they are firmly attached, recurved towards the base, and 

 culminate in an acute point (fig. 4). 



The beauty of this little ichthyodorulite has suggested its specific 

 name. 



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



It is possible that the small spine, P. pulcTiellus, may be the 

 immature form of some other species ; but I have at present no evi- 

 dence that such is the case. I have in all half a dozen specimens ; 

 and they are all of the same size within the eighth of an inch.^ It 

 most resembles P Icevissimus, Ag. There is a considerable simi- 

 larity in the denticulation of this form and the type of Agassiz ; at 

 the same time the method of insertion in the mass of the spine is 

 different. If these were young and immature specimens, we ought 

 also to find them of other and intermediate sizes ; but hitherto such 

 has not been the case. It is possible that further evidence may be 

 found ; but for the present it will be better to distinguish these 

 spines as a distinct species. 



5. Pleuracanthtjs alteenidentatfs, sp. nov. PI. XII. fig. 3. 



Spine straight, 2*5 inches in length when perfect, '2 inch in 

 greatest diameter at the base. Prom the base the diameter of the 

 spine decreases until it ends in a blunt point ; the internal cavity is 

 terminal, circular, and comparatively small ; external surface striated 

 longitudinally ; anterior and lateral faces circular in section ; the 

 posterior depressed and about the width of the diameter of the spine 

 (fig. 5). 



Pig. 5. — PleuracaniJms alternidentatus, Davis. 



M 



Section of spine, nat. size. 



The angle formed by the junction of the lateral and posterior faces 

 is set with seven or eight widely separated obtuse denticles, ex- 

 tending from the apex along one third of the length of the spine. 

 They present the peculiarity of being inserted alternately, the 

 denticle of one side being opposite to the depression on the other. 

 Prom this characteristic I have applied the nomen triviale as above. 

 Locality. Coal-measures, Middleton near Leeds. Leeds Literary 

 and Philosophical Society's Museum. 



