64 ECHINODERMS OF THE BRITISH ISLES 



marginal plates distinguishes this species from all the other 

 species of the genus Plutonaster. 



In British seas this species appears to be fairly common in 



Fig. 36. — Plutonaster bifrons, oral and aboral side. Somewhat reduced 



in size. 



deep water off the Irish coasts, especially in depths beyond 

 900 m. It is elsewhere distributed from the Faroe Channel to 

 about the Cape Verde Islands ; occurs also in the Mediterranean. 

 Known bathymetrical distribution, ca. 100-2500 m. 



5. Persephonaster Alcock. 

 (Syn. Psilasteropsis W. K. Fisher.) 



Disk rather small ; arms low, tapering to a point. Marginal 

 plates massive, somewhat tumid ; lower marginals with an 

 oblique series of slender spines. Dorsal side of arms very narrow 

 on account of the encroaching of the marginals. Oral inter- 

 radial areas rather small ; the interradial plates extend about 

 one-third the length of the ray. Madreporite naked ; simple 

 pedicellarise may occur. 



Only one species has been found in the British seas, but one 

 more, viz. Persej)1ionaster humilis (Koehler) (Syn. Psilasteropsis 

 humilis Koehler), may be expected to occur there also. This 

 latter species, known only from off the Azores, 845 m. (" Princesse 

 Alice "), is very easily distinguished from P. patagiatus by having 

 a large spine on the uj^per marginal plates. 



