BRITISH FOSSILS. 



3 



series, the three centro-lateral ones are very large and conspicuous ; 

 those between them and the mouth much smaller but still pro- 

 minent ; those between them and the apical disk very small and 

 inconspicuous. The areolse of the latter are but slightly depressed 

 and not well defined ; but the areolae of all are often confluent ; 

 when not so, it is the uppermost tubercles only that are separated 

 by granules. The avenues of pores are very slightly sinuous. 

 The pairs of pores are set rather obliquely in single file, except 

 near the mouth, where they fall into indistinct ranks of threes. 

 The mouth occupies about half the diameter of the under surface, 

 and is lodged in a concavity with rounded and tumid sides. 

 It is distinctly decagonal and ten-notched ; the margins of the 

 notches are strongly reflected. 



The apical disk is oblong, with the vent excentric and placed 

 far back, so as to encroach in some specimens on the posterior 

 ambulacrum. It is not prominent but rather plane. It is com- 

 posed of five genital plates, five oculars, and from three to six 

 supplementary plates, one of which is regular and sub-central. 

 All are ornamented with scattered granules surrounded by well- 

 marked areolae. The antero-lateral genital plates are large, 

 shield-shaped, and nearly regular. The postero-laterals are similar, 

 but somewhat oblique. Both bear their perforations sub-mar- 

 ginally within their projecting anterior angulated portions. The 

 posterior genital plate, bounding the vent, is very narrow and 

 semi-lunar ; it has the perforation distinctly marked. The oculars 

 are well-marked, broad, and rather large ; the three anterior ones 

 symmetrical ; but the two hinder ones are unsymmetrical, being 

 narrowed and produced posteally. In all, the eye-perforations are 

 lodged in a marginal depression. The supplementary or sur-anal 

 plates are polygonal ; the hinder ones smallest. 



The primary spines are sub-cylindrical, subulate, and to the eye 

 appear nearly smooth ; but under the lens they are seen to be 

 obsoletely striated. They are often slightly curved near their bases, 

 and have their neck-rings set obliquely. The longer ones exceed 

 somewhat in length the diameter of the body. The secondary 

 spines are very small, rather stouter in proportion to their length, 

 and more distinctly striated. 



The largest specimen which I have seen measured three-fourths 

 of an inch in diameter, had a disk three-tenths of an inch in 

 length by three-twelfths in breadth, and primary spines one inch 

 long. A smaller and more ordinary example is seven-twelfths of an 

 inch in diameter by four-twelfths in height. 



