106 ECHINODERMS OF THE BRITISH ISLES 



not continuous across the mid-line ; there is a small median 

 projection on the jaws. No spines on outer surface of jaws. 

 Adambulacral spines 5. The fin-like membrane inconspicuous, 

 scarcely visible from the dorsal side. R. = 2 r. Largest specimen 

 knowTi, 55 mm. R.^ Colour in life unknown. Biology unknown. 

 Only two specimens known, taken at 51° 1' N., 11° 50' W., 

 750 fms. C Flying Falcon "), and 51° 15' N., 12° 17' W., 673-893 

 fms. (■' Helga "). The species has also been recorded from the 

 Bay of Biscay (800 m., "Caudan"); but it seems doubtful 

 whether the specimen obtained there is really Pt. personatus. 



2. Hymenaster Wyv. Thomson. 



Supra-dorsal membrane with numerous muscle fibres, which 

 do not form a regular network. Adambulacral spines not united 

 by a web to form regular transverse combs. Tube-feet in two 

 series. Fin-like membrane usually very conspicuous. 



Two species known from British seas, but four more may well 

 be expected to occur there also, viz. Hymenaster rex Perrier, 

 known from the Bay of Biscay to S. of the Canaries (ca. 1140- 

 2300 m.) ; H. Giboryi Perrier, from off the Azores to off Portugal 

 (ca. 1900-4275 m.) ; H. roseus Koehler, from off the Azores 

 (ca. 1850-2100 m.), and H. membranaceus Wyv. Thomson, from 

 S.W. of Cape Finisterre (41° 58' N., 9° 42' W., 2025 m.). These 

 species are therefore included in the key. 



Key to the species of Hymenaster known from or likely to occur 

 in the British seas. 



1 . Three furrow spines ^ ....... 2 



Two „ „ 3 



2. Furrow spines slender, conspicuously decreasing in length from 



the proximal to the distal. Spines on outer surface of jaw 

 slender. A large naked interradial space extending more than 

 halfw^ay to the middle of the disk, the rays appearing straight - 

 sided in almost their whole length . . . H. rex Perrier 



Furrow spines short, robust, equal sized. Outer spine on outer 

 surface of jaw broad, compressed. Naked interradial area 

 less developed, raj^s gradually broadening proximally 



H. membranaceus W. Th. 



^ The size, 70 mm. R., given by Sladen appears to be a mistake 

 (according to Farran). 



2 Sometimes three furrow spines may be found also in H. pellucidus 

 and giganteus ; in such cases the descriptions of these species must be 

 consulted, as it would complicate the key too much to include such 

 exceptional cases. 



