2 



BRITISH FOSSILS. 



a little below the centre of the sides are equal to from three to three 

 and a-half of the ambulacral areas. In a specimen an inch and four- 

 twelfths high, about 15 plates constituted each half of a dorsal inter- 

 ambulacral area, and about 75 each half of an ambulacral. The 

 greater number of interambulacral plates are nearly equal in vertical 

 diameter, but widen out in transverse dimensions from apex to base- 

 The ambulacrals are minute, and often cuneiform, and are separated 

 from each other not unfrequently by small accessory plates. Rather 

 more than four of the former are equal to the length of the inter- 

 ambulacral plate opposite them. All the dorsal plates are covered with 

 very minute miliary granules, which give but very slight or scarcely 

 any roughness to the surface when it is denuded of spines. These 

 are interspersed with small spiniferous tubercles, numerous, scattered, 

 but presenting an hour-glass like arrangement on each interambulacral 

 plate, one or two on each ambulacral. Each of these spiniferous 

 tubercles is perforated on the apex, and placed on a mammilliform 

 boss with a crenulated summit, and surrounded by a smooth areola. 



The base is flat. Its ambulacral and interambulacral areas are 

 studded with numerous spiniferous tubercles larger than those of the 

 dorsal surface ; in the former they form oblique rows of fours or at 

 most five, across the whole of each ambulacral space near the margin, 

 and diminishing in number towards the mouth ; in the latter they are 

 thickly crowded towards the margin, and ranged towards the mouth in 

 oblong transverse groups, interrupted by raised granulated wavy 

 spaces, which mark the lines of junction of the plates, and forming an 

 arachnoid arrangement of irregular rings on the base, of which the 

 mouth is the centre. The base is elongated posteally, thickened and 

 subrostrated for the anus, which is large and broadly elliptical in a 

 longitudinal direction. Its margins are raised and thick. The size of 

 the vent usually exceeds that of the mouth by one-third. The mouth 

 appears round, but is obscurely decagonal, being gently notched at its 

 margins opposite the avenues of pores. It is inflexed opposite each 

 area, the inflections opposite the interambulacral areas being deeper and 

 semicircular. The pairs of pores are small, ranged in single file down 

 the dorsal surface, falling into series of three pairs on the base soon after 

 passing the margin. The series become more and more oblique until, at 

 the tenth or eleventh from the base, they are directly under each other, 

 and consequently give considerable breadth to the avenues. Three 

 lines drawn up each avenue in the inner half of its basal course will in- 

 tersect one of the pair of pores of each series. 



The miliary tubercles of the dorsal plates bear very minute and 

 short tubercular smooth spines, each standing apart from its neighbours. 

 The spines on the tubercles of the upper surface I have not seen. 



