294 ECHINODERMS OF THE BRITISH ISLES 



1. Psammechinus L. Agassiz. 



Test closely set with spines (tubercles) of nearly equal size, 

 the primary series not very conspicuous. Three pore pairs in each 

 ambulacral plate. Globiferous pedicellarise with outer part of 

 valves (the " blade ") rather broad, open, the edges thin and not 

 connected by cross beams on the inside ; a series of teeth along 

 each side of blade (Fig. 143, 2-3 ; 144, 1). Buccal membrane 

 usually closely packed, as if imbricated, with thick plates. Small 

 or moderate sized forms of greenish colour. 



One species known from the British seas. Another species, 

 Ps. microtubercidatus (Blainville), is very common in the Medi- 

 terranean, but occurs also on the Atlantic coasts to Cape Verde in 

 the south and the Portugal coast in the north. There is, 

 perhaps, a possibility that this species may occur as far north 

 as the British south coasts. 



Key to the species of Psammechinus known from or possibly 

 occurring in the British seas. 



Spines rather coarse, usually with violet point ; plates of buccal 

 membrane consisting of usual calcareous network, not greenish. 

 A rather robust form, up to 50 mm. horizontal diameter 



1. Psammech. miliaris (Gmelin) 

 Spines fine and slender, with reddish point ; plates of buccal mem- 

 brane of compact, greenish calcareous substance, with only 

 some tube-shaped holes in them. A rather delicate form, 

 usually not exceeding 35 mm. horizontal diameter 



Psammech. rnicrotuherculatus (Blainv.) 



1. Psammechinus miliaris (Gmelin). (Fig. 166.) 



(Syn. Echinus miliaris ; Par echinus miliaris (Miill.).) 



Test rather depressed, often subpentagonal in circumference, 

 closely set with rather robust, short spines, the primary ones 

 being not very conspicuous ; on the naked test the series of 

 primary tubercles are, however, fairly distinct. There is a 

 primary tubercle on every ambulacral plate. Apical system 

 rather small, none of the ocular plates in contact with the 

 periproct. The buccal plates with numerous pedicellarise, but 

 with no spines. The plates of the buccal membrane of usual 

 reticulate structure, not greenish. Globiferous pedicellarise 

 (Fig. 143, 2-3 ; 144, 1) small, usually very numerous, looking 

 with their white heads, on well preserved specimens, like a down 



