/. — Pleuracanthidce. 741 



an acuminate apex. In section the spine is more or less round, except at the base, 

 which is compressed antero-posteriorly ; the extremity is slightly tapering and 

 rounded. The internal cavity is not terminal, but is open along the posterior 

 surface for a distance of about 0'04m. The oriiice is large and the walls thin at 

 the base ; higher, the cavity is reduced to one-third the diameter of the spine, and 

 gradually diminishes towards the apex. The surface of the spine is covered with 

 longitudinal striations which sometimes disappear towards the apex. On the 

 posterior surface is a double row of closely approximating denticles ; they extend 

 over one-half the length of the spine, and number seventy or eighty denticles on 

 each side ; they increase in size with the diameter of the spine or towards the base. 

 The denticles are round near their base, contracting to an obtuse point, directed 

 diagonally towards the base, and at the same time away from the centre, of the 

 spine (fig. 1). 



A magnificent specimen of the spine of this species is preserved in the British 

 Museum (Nat. Hist.). It forms a part of the Egerton collection, but unfortunately 

 there is no record of the locality from which it has been obtained. The matrix is 

 a hard ironstone shale, and there are a number of moUuscan remains on the slab, 

 Goniatites, Pecten, and other marine forms, together with the remains of a plant, 

 2i^^2ive\it\y Lepidosirohus. The spine is 0'39 m. in length, and the base is imperfect. 

 Its greatest diameter is 0"018 m. The spine has a slight, graceful curvature, with 

 smooth surface somewhat deeply striated longitudinally. The denticulated surface 

 extends O'lSm. along the posterior surface, and on each row there are fifty denticles. 

 Those situated on the upper part are long recurved hooks, each separated from the 

 next by a distance equal to the diameter of its own base. Midway along the den- 

 ticulated surface the denticles are larger, thicker, and stronger ; at the lower part 

 they diminish again in size and are shorter and more stumpy, where not broken 

 off in the opposing matrix of the opposite slab. (PI. lxxiii., fig. 2). 



The largest examples of this species are from the Fenton and Knowles Ironstone 

 shales of North Staffordshire. The specimens from the Scotch Measures are smaller, 

 and some of them, as, for example, one from Quarter Hamilton (fig. 4), in the col- 

 lection of Mr. James Thomson, of Glasgow, has the rows of posterior denticles 

 situated wider apart than those described, the intervening area being quite convex, 

 whilst in those from Staffordshire it is flat or slightly concave (fig. 3). 



The spine of Orthacanthus hohemicus, Fr., from the gas-coal of Nyran, in 

 Bohemia, probably occupies a position closely allied to this species. 



Formation and Locality. — Shale above the Ragmine Ironstone, Fenton ; Knowles 

 and Chalky-mine Ironstones, Longton ; and Brown-mine Ironstone, Silverdale, in 

 Staffordshire ; Quarter Hamilton, Scotland. 



Ex coll. — John Ward, Longton; James Thomson, Glasgow; Egerton Collection, 

 Natural History Department, British Museum. 



