BRITISH FOSSILS. 



3 



arranged in ascending oblique rows ; and, on the centro-sutural side, 

 in horizontal rows. Towards the margin the tubercles become more 

 numerous, and sometimes less regularly disposed. The plates of the 

 ventral surface are of equal vertical dimensions with those of the dorsal, 

 and similarly ornamented. In well-grown specimens they are from 

 six to eight in number. The suture of articulation of the interambu- 

 lacral plates with each other varies in degree of straiglitness, or zigzag, 

 in different specimens ; and is most nearly straight when the dorsal 

 surface is least depressed. 



The ambulacral plates are very numerous and narrow, four of them 

 going to the vertical diameter of an interambulacral plate. They are 

 closely studded with granules like the latter, and each bears usually 

 one primary tubercle. These tubercles are so arranged that they do 

 not fall in direct vertical rows, but form oblique series of threes, or, 

 as in some specimens, where alternate plates have their primary 

 tubercles suppressed, in twos. On the ventral surface the tubercles 

 are similar, except that they are often larger than those on the 

 dorsal. Each plate corresponds to a pair of pores in the ambulacral 

 avenue. The avenues are impressed, quite straight, and have the pairs 

 of pores in regular succession, single file. They ha-ve a tendency to 

 become obsolete near the mouth. The two pores of each pair are 

 obliquely set with respect to each other. As there are about 21 inter- 

 ambulacral plates in each vertical series, from mouth to apex, in an 

 ordinary specimen, the number of ambulacral plates will be about 84, 

 and that of pairs of pores the same. The primary tubercles are placed 

 on elevations in areolated spaces, and are invariably perforate. 



The mouth is circular, and occupies the centre of the more or less 

 concave ventral surface. Its breadth is rather less than the distance 

 from its side to the outer margin. The anus occupies a considerable 

 part of the ventral space between the two postero-lateral ambulacra and 

 the margin. It is of an ovate form, its greatest dimensions being 

 in the line of length, i. e., from the mouth towards the margin, and its 

 outer extremity is slightly pointed. 



The apex is composed of the combined ovarian and ocula r plates, and 

 is in great part occupied by a madreporiform tubercle. The posterior 

 ovarian pore is undeveloped, as in all the genus. The ocular pores 

 are distinct. 



Casts of the interior differ from the external shape of the body in 

 exhibiting ten deep notches around the margin, extending to the mouth 

 on the ventral surface, and ceasing at about a third of the height on the 

 dorsal. Five of these unite near the mouth, and five continue singly to 

 it. The centre of the prominent interspaces of the latter is marked by 

 a shallow groove. The groove corresponds to the line of suture of the 



