ECHINODERMS OF THE LONDON CLAY. 



Sub-Kingdom. — RADI ATA. 



Class.— ECHINODERMATA. 



Family. — Cidaeitid^e. 



The members of this family are distinguished among those which have the vent and 

 mouth at opposite poles, and the former aperture surrounded by the genital and ocular 

 plates, by the pairs of pores in their avenues being ranged in true single file, and by their 

 narrow ambulacra, upon which are no primary spines. They have a well-developed dental 

 apparatus. Of all the families of Echinidea this is the most ancient, certain palaeozoic sea- 

 urchins appearing to be truly species of Cidaris. 



Genus — Cidaris, Lamarck. 



Test turban-shaped, thick, the ambulacral areas very narrow, and bearing secondary 

 tubercles and spines only; its interambulacral segments broad, ornamented with large and 

 few perforated tubercles, placed on smooth or crenulated bosses, and bearing variously 

 shaped strong spines, always different in form and sculpture from the secondary spines. 

 Pores of the avenues in strict single file. Oral membrane covered with imbricated scales. 

 Eye-plates and genital plates all perforated. 



The existing species of Cidaris are distributed through the seas of all regions, but the 

 majority are congregated within the tropics. The number of mesozoic species was much 

 more considerable. Of tertiary forms our knowledge is not very precise ; and unfor- 

 tunately, as in the instance about to be noticed, confined to their spines only in too many 

 cases. 



