274 ECHINODERMS OF THE BRITISH ISLES 



the various recent forms described cas Porocidaris species have 

 any nearer relation to the fossil Porocidaris. The genus Porio- 

 cidaris is very nearly related to the genus Histocidaris Mrtsn., 

 and may perhaps not deserve to rank higher than as a subgenus 

 of Histocidaris, from which it differs only in its peculiar bivalve 

 jiedicellaria} (in Histocidaris they are usually three- valved, but in 

 one species also bivalve pedicellariae occur) and in the un- 

 usually long collar of the primary spines. 



Only one species in the British seas, the only species known. 



1. Poriocidaris i^urpurata {yVyv. Thomson). (Fig. 154.) 



(Syn. Porocidaris 2^ur2)urata Wyv. Thomson ; Cidaris jiurpurata 

 (Wyv. Thoms.) Bell ; Histocidaris purpurata (Wyv. Thoms.) 

 H. L. Clark ; Porocidaris gracilis Sladen.) 



Ambulacra! areas rather closely tuberculated, the tubercles 

 forming, however, four fairly distinct longitudinal series. Apical 

 system about half the horizontal diameter of the test ; oculars 

 sometimes partly in contact with the periproct. Female genital 

 pores very large, may even break through the outer edge of the 

 genital plate. Primary spines slender, with more or less numerous 

 rather coarse thorns ; they are about twice as long as the 

 diameter of the test. In adult specimens the upper spines are 

 much shorter, very pointed, their collar often very long and more 

 or less swollen.^ Colour in life brownish -violet ; especially the 

 collar of the primary spines usually of a dark purplish -violet 

 colour, which forms a striking contrast to the white or pale pink 

 outer part of the spine. Reaches a size of ca. 50 mm. diameter 

 of test. 



Development unknoAvn, but the large size of the eggs and 

 genital openings indicate that it has direct development, without 

 a pelagic larva. There are no observations to support the 

 suggestion that it might protect its brood. It lives on muddy 

 bottom. 



This very beautiful species was first discovered north of the 

 Hebrides (" Porcupine ") ; it has since been found several times 

 off S.W. Ireland (" Helga "), in depths of ca. 1000-1400 m. It 



^ A form with the collar of the primary spines unusually swollen was 

 designated by the author (" Inqolf " Ech., i.) as a separate variety, the var. 

 talismani. There is, however, no reason to maintain this variety, as all 

 degrees of swollen collars may be found. The Porocidaris gracilis of Sladen 

 is only the young P. purpurata. 



