BRITISH FOSSILS. 



These ambulacral tubercles are separated from each other by narrow 

 and irregular rows of granules, and are so arranged, in consequence 

 of being closely set, that the tubercles of each row alternate. The 

 ambulacral segments are each composed of two series of six or 

 seven plates, each plate bearing a primary tubercle. The two plates 

 nearest the mouth bear small ones ; those of the sides very large 

 ones, and those near the apical disk small ones. The tubercles 

 are elevated on very prominent bosses (to the size of which 

 their apparent dimension is chiefly due), around whose bases 

 there is a broad smooth areola. The tubercles are perforate ; 

 the summits of the bosses crenulate ; the number of crenula- 

 tions ten or more. The areolae are confluent, separated by a 

 single line of minute granules. Alternations of minute secon- 

 dary tubercles and small granules closely set form a narrow 

 border to the ambulacral and interambulacral margins of each plate. 

 Thus the medial region of each interambulacral segment becomes 

 ornamented by a sinuous line of irregular secondary granules, two 

 abreast, except above, where they become rather more numerous, 

 until near the genital disk they become scattered or obsolete. 

 Their number is slightly increased in the ambulacral margins of 

 the interambulacral plates near the mouth. The mouth is wide, 

 usually equalling in width the height of the test. Its margins are 

 decagonal in consequence of ten deep notches with reflected edges 

 that indent in pairs the buccal extremities of the interambulacral 

 segments. The interspaces of these notches describe arches ; those 

 of the ambulacral segments being widest. The avenues of pores 

 are very slightly sinuous ; above, they are in strict single file ; near 

 the mouth they fall into two or three somewhat irregular curved 

 ranks of three pairs in each. There is a small swelhng or granule 

 separating the pores of each pair. 



The apical disk is wide and slightly convex ; it occupies about a 

 third of the diameter of the test. It is slightly pentagonal in 

 consequence of the projection of the genital plates. These are 

 large and escutcheon-shaped, except the posterior one, which is 

 contracted in consequence of the excentric and posteal position of 

 the vent, of which it forms one of the bounding plates. The genital 

 orifices are placed not far from their outer margins. The right 

 antero-lateral genital plate bears the madreporiform tubercle, in- 

 dicated by a porous subcentral space. The eye plates are large, 

 but the ocular pores are concealed in sub-marginal depressions. 

 The^centre of the disk is occupied by from four to six supra-anal 



