2 ECHINODERMS OF THE CRAG. 



conjunction with the plates bearing the eyes. These latter are always at the superior 

 termination of the ambulacra. They move by suckers, which are extended from the 

 ambulacral pores, and assist themselves in their movements by spines, which are articulated 

 to tubercles projecting from the surface of the plates of the test. 



Family. — EchiniDjE. 



Three of the families of the Sea-Urchins are distinguished from all others, in having 

 their anal orifice placed in the centre of the genital plates, and directly opposite to the 

 mouth, which occupies the centre of the ventral surface. Of these groups, the Palachinida 

 are peculiar to Palaeozoic strata; the Cidaritida and Echinidce range from the commence- 

 ment of the Mesozoic epoch to the present day. No Cidaris has, as yet, occurred in the 

 British Crag. In the collection of Sir Charles Lyell are the spines of a very distinct 

 species from the Black Crag of Antwerp. The Echinida are distinguished by their 

 developed ambulacral areas, always bearing some or many primary spines, and by their 

 rows of pores ranged in more or less distinct ranks. 



Genus — Echinus, Zinnaus. 



Body more or less spherical; ambulacral and interambulacral segments developed, 

 bearing on their plates, which have entire margins, tubercles of various sizes; vent 

 centrical, not furnished with regular calcareous valves, but surrounded by a membrane 

 covered more or less with irregular ossicles. Genital disk surrounding the anal space, 

 and composed of five genital and five ocular plates, all perforated and alternating ; one of 

 the former combined with a madreporiform tubercle ; ambulacral avenues composed of 

 pairs of pores, ranged in series of three or more, always distinctly ranked near the mouth ; 

 spines of one order. 



All the species of Crag Echini belong to the typical section of the genus, that in which 

 the pores are ranged in ranks of three pair in each. Such forms are, at present, pre- 

 eminently of Atlantic types. It is remarkable, that none of the common British species of 

 this genus can be identified with those found in the Crag. 



1. Echinus Lamarckii. Plate I, fig. 4. 



Very reluctantly, I feel obliged to designate, by a new name, the commonest of sea- 

 urchins found in the Coralline Crag. It is very closely allied to Echinus sphara, the most 

 abundant species in the Celtic province of the Atlantic ; so nearly, indeed, that not until 

 after considerable hesitation, did I feel constrained to regard it as distinct. It differs 



