ECHIXOIDEA 



279 



3. 



Key to the genera of Echi not hur idee known from the British seas 

 {and the N.E. Atlantic). 



Primary spines of oral side club-shaped, enclosed in a rather 



thick bag of skin ... 1. Phormosoma Wyv. Th. 



Primary spines of oral side ending in a white hoof . . 2 



Tube -feet on oral side arranged in a single, more or less irregular 



series (Fig. 156, 2) . . . 3. Hygrosoma Mrtsn. 



Tube-feet on oral side arranged in three series ... 3 



The larger primary plate in each compound ambulacral plate 



divided in two parts, each compound ambulacral plate thus 



composed of 4 plates, one of them without pores ; all the pore 



plates of about equal size (Fig. 156, 3) 4. Sperosoma Koehler 



Each compound ambulacral plate consists of 3 plates only, the 



large, undivided middle plate and two very small plates 



enclosed between the larger primaries, not reaching the edge 



of the area (Fig. 156, 1) . . .2. Arceosorna Mrtsn. 



Fig 



156. — Part of ambulacra of Arceosorna hystrix (1), Hygrosoma Petersii 

 (2), and Sperosoma Grimaldii (3). Slightly diagrammatic. 



1. Phormosoma \Vw. Thomson. 



Test rather rigid, flat below, gently arched on the upper side. 

 Oral side with numerous large tubercles (areoles) and spines, the 

 upper with only few of them, and much smaller ones, which gives 

 the two sides a very different aspect. Edge of disk rather sharp. 

 Tube -feet arranged in a single series on the oral side. Primary 

 spines on oral side club-shaped, enclosed in a rather thick bag 

 of skin. Only tridentate and triphyllous pedicellariae. 



