320 ECHTNODERMS OF THE BRITISH ISLES 



other plates, forming a sort of ventral shield, a " sternum " 

 or a "plastron" (Eig. 185, 1). Eascioles usually well 

 developed. 



The fascioles are very diversified within this suborder and are 

 of great importance for the classification. The following forms 

 of fascioles are distinguished: " peripetalous'\ which surrounds 

 the whole of the petals on the upper side (Fig. 200, 1) ; " inner ", 

 surrounding only the anterior ambulacrum on the upper side 

 (Fig. 194) ; " lateral ", issuing from the peripetalous fasciole and 

 running along the sides of the test backwards under the peri- 



1 2 



Fig. 185. — Amphisternous (1) and meridosternous (2) type of sternum 

 (or " plastron "). 



1. Of Spatangus purpureus ( x 3). 2, Of Urecfiinus naresianus ( x 6). 



proct (Fig. 189) ; '' subanaV, forming a closed ring below the 

 periproct and enclosing a certain number of penicillate tube-feet 

 (Fig. 200, 2) ; " marginal", running all round the test at the margin. 

 The small spines (" clavulai ") of which these fascioles consist are 

 very strongly ciliated. 



The forms belonging to this suborder have been divided 

 (Clark) into five families, two of which are represented in the 

 British seas, while two more are very likely to occur there 

 also.^ 



1 The fifth family, the Palceostomatidce, contains only a single, small 

 species, Palceostoma mirahile Gray, from the Indo-Malayan region. — It 

 may be questionable whether the families of the Amphisternata are quite 

 natural, but for the present this classification may be accepted as the best 

 and most practical. 



