102 



ECHINODERMS OF THE BRITISH ISLES 



Key to the genera of Pterasieridce known from or likely to be 

 represented in the British seas. 



1. Tube-feet in four rows .... Diplopteraster YerriW 

 Tube -feet in two rows ....... 2 



2. Adambulacral spines united by a web so as to form a transverse 



comb (Fig. 58) . . . 1. Pteraster Miill. & Troschel 



Adambulacral spines not united by a web .... 3 



3. Mouth plates large, with spines on outer surface ; fin-like mem- 



brane very conspicuous . 2. Hymenaster Wyv. Thomson 



Mouth plates small, without spines on outer surface ; fin-like 



membrane indistinct .... Cryptaster Perrier 



1. Pteraster Miiller and Troschel. 

 (Syn. Hexaster Perrier.) 

 Supra -dorsal membrane with muscles slightly developed, not 

 forming a regular network. Adambulacral spines united by a 

 web so as to form regular transverse combs (Fig. 58). Tube-feet 

 in two series. Fin-like membrane usually well developed. 



Two species are known from the British seas, but very prob- 

 ably some of the following species 

 wdll prove to occur there also, viz. 

 Pteras^er^ pulvillus M. Sars, known 

 from the Scandinavian seas to Spitz - 

 bergen and the Siberian Sea, as also 

 from Greenland to 42° N. on the 

 American coast, and from the 

 Bering Sea (ca. 15-400 m.) ; Pt. 

 reductus Koehler, known from off 

 the Azores (1846-2870 m.) ; PL 

 sordidus Perrier, known from off 

 Morocco (1 139 m.), and Pt. alveolatus 

 Perrier,^ known from off the Azores 

 (4060 m.). These species are therefore included in the key. One 

 more species may be mentioned, viz. Pteraster ( Hexaster) obscurus 

 Perrier, differing from all the other species in having 6 arms ; 

 as it is not known to the S. of Finmark in European seas, there 

 is not much prospect of its occurring as far south as the British 

 seas, but it is perhaps not excluded that it may be found in the 

 cold area of the Faroe Channel. It is known elsewhere from 

 Greenland to the Bank of Newfoundland, and from the Bering 

 Sea (30-450 m.). 



^ Sladen (" Challenger " Asteroidea) refers this species to his genus 

 Marsipaster ; judging from the description and figures given by Perrier 

 {Echinodermes du " Travailleur '" et du "Talisman''), it seems to be a 

 true Pteraster. 



Fig. 58. — Adambulacral spines 

 of Pteraster militaris. The 

 long lateral spines support 

 the fin-like membrane along 

 the edge of the disk. x 8. 

 (From Danmark'' s Fauna.) 



