BEITISH FOSSILS. 



5 



differences in the number of the plates of the inferior surface are not 

 so great. 



The ambulacra are, dorso-laterally, at about the centre of the sides, 

 in breadth to the interambulacra as 1-3. This proportion varies 

 a little in different specimens, but without much regard to the 

 general contour. Except in size, they exactly resemble the ambu- 

 lacrals. On the upper third of the test they are small, short, and 

 closely set ; then gradually become taller down the sides and as 

 they approach the margin. They and the interambulacrals are 

 similarly ornamented with minute scattered primary tubercles, 

 each of which is perforated and set upon a boss with a crenulated 

 summit. On a large interambulacral plate in one of the rougher 

 varieties I have counted as many as fifty tubercles, and yet they 

 appear wide apart. The interspace is studded with scattered 

 miliary granules forming imperfect rings around the tubercles 

 In an individual of one of the smoother varieties there may not be 

 more than about fifteen tubercles upon the corresponding plate. 

 The pairs of pores occupy each an ambulacral plate, and are placed 

 near its centre, slightly oblique, a very shallow groove connecting 

 the one pore with the other. In consequence of their closeness 

 they are conspicuous near the apical disk, but are inconspicuous on 

 the sides. 



The base varies in degree of flatness, and specimens are occa- 

 sionally found remarkably rounded at - the margins ; usually, how- 

 ever, it presents an extensive flattened surface. At one extremity 

 of it is seen the longitudinally elliptical vent, near the other the 

 transversely oval mouth. The space between the mouth and vent 

 is plane, and occupied by the subanal plates of the posterior inter- 

 ambulacral segment. There are about six of these plates in each 

 alternate row, and their inner margins are curved hindwards. 

 They are covered with tubercles, thickly set, and consequently con- 

 trasting with the bordering inferior ambulacral plates, which are 

 minutely granulated but not tuberculated, and present the aspect of 

 a lanceolate rough space or elongated escutcheon, the base of which 

 is formed by a raised ovate leaflike space in front of the vent, 

 marking the position of internal buttresses. The tubercles on the 

 anterior portion of this space are smaller than those of its hinder 

 half and than those of the escutcheon that embraces it. They are 

 also more scattered, and are more separated by minute granules. 

 The postero-lateral ambulacrals in their basal portion are elongated 

 to reach the mouth, and their plates become longer and somewhat 



