176 



CLYPEASTERIyE. 



and at the same time one of the commonest and prettiest. 

 When alive it is of a bright green colour, such a powdery 

 green as is seen on the elytra of many beetles ; but when 

 cast on the shore, as we usually find it, dead, it becomes of 

 a dirty white. It is very common among shell sand. 



The upper surface is depressed ; the general form ovate, 

 rather broader posteriorly than anteriorly. In the centre 

 of the back are seen the four ovarian holes, and from these 

 radiate the five ambulacra, at nearly equal distances from 

 each other, each composed of two double parallel rows of 

 seven or eight pairs of pores. These ambulacra run about 

 half way towards the circumference. They are level with 

 the surface, which is studded with little flattish moniliform 

 spiniferous tubercles, which are surrounded by smaller 

 ones. Beneath, we find the mouth in the centre, which 

 is somewhat concave, and between it and the broadest 

 margin is the anus. Both are round, the latter much 

 smaller than the former. The spines are very short, close- 

 ly placed, and resembling those of Sjmtangus in their 

 structure. The coats of its intestine are covered with 

 vibratile cilia. 



Internally the shell is strengthened by ten strong ribs, 

 which run in pairs at equal distances from mouth to apex.* 

 The mouth bears a dental apparatus of the same nature 

 with that seen in Echinus:, but different in form, being 

 much shorter and broader, as befits the shape of the in- 

 ternal cavity. It consists of five complicated teeth, the 

 keels of which are greatly produced into compressed beaks, 

 which are rounded at the margins and grooved. The 

 whole apparatus has the appearance of five rostra placed 

 round an ancient naval column. 



The Green Pea-Urchin has been observed on most parts 

 of the coast of Britain from Devon to Shetland, both east 



