PENTAOEROS BISPINCXSUS. 87 



The supero-marginalia and infero-marginalia are equal in number. There 

 were probably thirteen of each in the space between an interradius and an 

 extremity of an arm. 



In the interbrachial arc there is a series of minute granular intermarginalia. 



The traces of the disc which are present suggest that the ossicles of this region 

 were oval in shape and minute in size. I exposed a portion of the ventral surface 

 of the arm, but, unfortunately, little trace of structure was shown. The ventro- 

 lateralia extended to the extremity of the ray. The ridges of the adambulacral 

 armature are lost. 



Locality and Stratigraphical Position. — Upper Chalk, Charlton, Kent. 



7. Pbntaceros bispinosus, n. sp. PI. XXIII, figs. 3, 3 a, 3 b, 3 c. 



Specific Characters. — Disc large. Arms moderately produced. Single isolated 

 marginal ossicles vertebra-shaped with biconcave extremities. Ventro-lateral 

 plates with strongly marked sockets for two or more spines. 



Material. — The only specimen of this species is that preserved in the British 

 Museum of Natural History (H. W. Taylor's Coll., 35482). Only the ventral 

 surface is exposed, and this is very much distorted. 



Description. — The disc appears to have been large. Its actinal surface is 

 covered with a number of sub-equal oblong or polygonal plates, which possessed 

 sockets in which fitted spines (PI. XXIII, fig. 3c). These plates are 4*8 mm. 

 long, and 3*1 mm. wide. 



The arms are moderately broad, and at least four series of ventro-lateral plates 

 enter at the base. R : r : : 60 mm. : 20 mm. (approximately), the major radius 

 therefore measuring about three times the minor radius. The marginal ossicles 

 are shaped very much like the centrum of a vertebra, and are biconcave. They 

 possess a distinct granulation in their central region, which is surrounded by a 

 wide margin. The infero-marginals at the base of the ray are about 3'2 mm. wide 

 and 2 - l mm. long. There were probably sixteen of them from the interradius to 

 the extremity of the ray. 



The specimen is otherwise so distorted that little can be made of its structure. 



Locality and Stratigraphical Position. — Upper Chalk, Sitting-bourne, Kent. 



