306 ECHINODERMS OF THE BRITISH ISLES 



length of head. Colour (in a ])reserved state) white. Not known 

 to exceed ca. 40 mm, horizontal diameter. 



This species has a considerable resemblance to Echinus 

 acutus, Var. norvegicus, but is easily distinguished from it by 

 its entirely white colour. Also the arrangement of the primary 

 ambulacral tubercles is different, there being here, in spite of the 

 irregularity in the size of the tubercles, a primary tubercle on 

 each ambulacral plate (as seen from the fact that they are placed 

 in a regular series), not only on every second or third plate as 

 in norvegicus. 



Biology and development unknown. Two species of parasitic 

 molluscs have been found infesting this species, viz. Pelseneeria 

 profunda Koehler and Vaney, and P. media Koehler and Vaney. 



From British seas this species has only been recorded with 

 certainty from off the Rockall Bank, 1750 m. (" Valdivia "). No 

 doubt it will be found to occur off the west coast of Ireland 

 and Scotland. It is known elsewhere from S. of Iceland to the 

 Azores, and also from off the east coast of N. America. The 

 bathymetrical distribution is ca. 770-2180 m. 



3. Paracentrotus Mrtsn. 



Test rather closely covered with spines (tubercles), among 

 which the primaries are fairly conspicuous. Ambulacral plates 

 with 5-6 pore pairs each (polyporous). Buccal membrane 

 with only small plates outside the buccal plates, thus rather 

 naked. Globiferous pedicellarise with only 1 lateral tooth to 

 each side, beneath the end tooth ; the blade not tube-shaped, 

 without transverse beams across the inside (Fig. 176, 1). 



Only one species in British (and European) seas. 



1. Paracentrotus lividus (Lamarck). (Fig. 175.) 



(Syn. Toxopneustes lividus (Lmk.) ; Strongylocentrotus 

 lividus (Lmk.).) 



Test rather low, but rounded. The primary tubercles form 

 very regular, fairly conspicuous longitudinal series, the secondary 

 tubercles smaller, not very numerous. Every ambulacral plate 

 carries a primary tubercle. Primary spines fairly long and 

 robust. Globiferous pedicellarise (Fig. 176, 1) without glands on 

 the stalk. Tridentate pedicellarise with narrow, slender valves, 

 with the edge distinctly serrate (Fig. 176, 2). Colour dark violet 

 to brownish or olive -greenish ; the test greenish. Does not grow 



