J. W. BATIS ON THE GEXTJS PLETJEACANTHTIS, AGASS. 327 



3. Plethiacai^^thxjs tenths, sp. nov. PI: XII. fig. 1. 



Spine remarkably long in proportion to the diameter ; the basal 

 and apical extremities are wanting ; when perfect, it probably 

 measured 6 inches in length. It is -2 inch in diameter. The basal 

 half of the spine is rotund in section ; on the remaining portion 

 the anterior and posterior faces are depressed, and their connexion 

 with the sides forms a right angle, so that the form of the spine 

 nearly approaches a square (fig. 3). Extending along the upper half 



Fig. 3. — PleuracantJms tenuis, Davis. 



& 



Section of spine, nat. size. 



of each lateral face is a row of blunt denticles tipped with bright 

 ganoine. The surface is covered with fine longitudinal striations and 

 a great number of minute punctures, which together give it a re- 

 ticulate appearance. There is an internal cavity, wide and round at 

 the base, and smaller near the apex. The cavity extends the whole 

 length of the part preserved. The spine probably converged to a 

 point when perfect. 



Locality. Bone-bed, Better-bed Coal, Clifton near Halifax 

 (L. CM.) 



Pleuracanthus tenuis is a sufficiently peculiar species ; the great 

 length, small diameter, and slightly curved form are characteristics 

 which at once distinguish it from aU other species having the 

 denticles arranged on the opposite lateral faces of the spine. It 

 very nearly approaches in form the spine of the recent Trygon, 

 the body of the spine being nearly square in section, and tapering 

 very little until the apex is reached. The teeth in the recent 

 form are long, pointed, recurved towards the base, and extremely 

 close together ; in the fossil one they are rather widely separated, 

 broad at the base, and end in an obtuse point. It is, further, the 

 only species having lateral rows of denticles which is curved. 

 It is not an uncommon occurrence to find spines approaching more 

 or less to the form P. cylindrims (Ag.) — that is, with the teeth 

 on the posterior aspect, which are curved. Several examples arc 

 known both in this country and America, but none, so far as I know, 

 of the P. Icevissimus type. 



4. Pletjeacanthus pulchellits, sp. nov. PI. XII. fig. 2. 



Spine small and straight, 1-5 inch in length, and -1 inch broad 

 at the base. Its breadth, for about three fourths the length is 

 nearly uniform ; it then becomes rapidly acuminate, and ends in a 

 point. Anterior and posterior faces smooth and considerably de- 

 pressed, the diameter from back to front being equal to half the 

 transverse diameter. An oval cavity extends from the base internally. 

 The part of the spine implanted is small, its waUs thin and fre- 



