248 Proceedings of the Royal Society of Edinburgh. [Sess. 
spines, it presented an unusually depressed appearance, the maximum 
diameter being 10 cm., the height 5 cm. Yet the hexradiate symmetry 
was so little apparent that it had escaped notice. 
On the test denuded of spines the presence of six ambulacra and 
six inter- ambulacra was very marked, depression in the inter-ambulacral 
areas giving the ambitus an approach to regular hexagonal shape. A 
peculiar feature is common to all the inter-ambulacral areas. These, 
instead of narrowing gradually towards their subtending genital plates, 
widen out about a centimetre from the summit, so that the recently formed 
plates are as large as, or even larger than, some of their predecessors. On 
Fig. 3. — Hexamery in Echinus esculentvs. Apical area x 2^ ; m, madreporite. 
the aboral surface hexamery is apparent in the peristome. Six teeth are 
present, the lantern itself is six-partite, the elements of the parts being 
normal, while the buccal tube feet are arranged in six pairs. 
The apical system, while presenting a hexradiate appearance, is less 
regular. The genital plates are five in number. Those in areas 2, 3, and 4 
(according to Loven’s system, and assuming that the madreporite retains its 
normal position in area 2) are normal in shape and position. Genital plate 
5 has a greater lateral diameter than usual ; opposite area 5 it is normal 
in shape, but it is continued for a short distance opposite area 6. The 
remainder of the apical system is filled in by a double plate which subtends 
both of the areas 1 and 6. This is practically equivalent to two normal 
plates laterally adnate. The genital pores are normal in position, there 
