118 ECHINODERMS OF THE BRITISH ISLES 



" Caudaii '"), and H. ahyssalis (Perrier) (off Morocco and the 

 Azores, ca. 1100-2165 metres). These species are included in 

 the key. ^ 



The species of this genus are mostly exceedingly variable and 

 the limits of the species very hard to fix ; hybridisation prob- 

 ably accounts for a good many of the intermediate forms. It is 

 by no means certain that all the above-named five Atlantic 

 species can be maintained ; especially it may be expected that 

 a richer material will prove H. caudani to be identical with 

 H. ahyssalis, the difference in the furrow spines given in the key 

 as distinguishing these two species being probably due more to 

 different mode of description than to really existing differences.^ 



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

 in the British seas. 



1. One or more distinct series of larger plates outside and parallel 



to the adambulacral plates. Spines on the outer surface of 

 the adambulacrals abruptly smaller than the furrow spines 2 

 Generally no distinct series of larger plates outside the adambu- 

 lacrals. Spines of the outer surface of adambulacrals gradually 

 diminishing in size from the furrow spines outwards, without 

 any abrupt difference in size 1. H. sanguinolenta (O. F. Miill.) 



2. Rays much swollen in the basal part, and here on each side with 



4-5 regular longitudinal series of plates, above the two series 



following the adambulacrals . . H. biscayensis (Koehler) 



Rays cylindrical ; no distinct longitudinal series of plates above 



the series following the adambulacrals .... 3 



3. Rays exceedingly slender, cylindrical ; only one distinct series 



of larger plates outside the adambulacrals 



2. H. abyssicola (Norman) 



Rays not especially slender, tapering ; 2-3 distinct series of larger 



plates outside the adambulacrals ..... 4 



4. Skeleton of dorsal side very close ; 1-2 furrow spines and four 



similar spines outside these . . H. caudani (Koehler) 



Skeleton of dorsal side more open ; 5-6 furrow spines, united by 



a small web so as to form a small comb H. ahyssalis (Perrier) 



1. Henricia sanguinolenta (0. F. Miiller). (Fig. 70.) 



(Syn. Crihrella sanguinolenta; Echinaster sanguinolentus (O. F. 

 Miill.) ; Echinaster scrohiculatus Dan. and Koren ; Crihrella 

 oculata (Pennant) Forbes ; Mcujdalenaster arcticus Koehler.) 



Rays more or less slender, tapering. Spines on the adambu- 

 lacral plates generally forming a single transverse series, diminish- 



^ The author has not had the opportunity of examining specimens of 

 these two species, or of H. biscayensis. 



