ASTEROIDEA 71 



The alternating arrangement of the niargmal plates make the 

 arms more flexible ; this fact, combined with the existence of 

 strong dorsal arm muscles, might lead to the suggestion that these 

 sea-stars may, perhaps, be able to make swimming movements. 

 There are, however, no observations to support such a suggestion. 



Only one family belongs to this suborder. 



I. Family Benthopectinid^ 



This family is distinguished by the characters of the suborder 

 as well as by having the papula? confined to a limited, often 

 specialised, area, the " papularium ", at base of the rays and 

 adjacent portion of the disk (or the whole disk). Intestine, 

 intestinal caeca, and anus present. 



Two genera represented in British seas, but one more, 

 Pectinaster Perrier, may prove to occur there also, the species 

 Pectinaster Filholi Perrier being known from the Azores to Cape 

 Verde Islands, ca. 1250-2330 m. (" Talisman "). This genus, 

 therefore, is included in the following key : 



Key to the genera of Benthopectinidce known from or likely 

 to occur in the British seas. 



1. A prominent odd interradial marginal plate, the upper usually 



with a very prominent spine . 2. Benthopecten Verrill 



No odd interradial marginal plate ..... 2 



2. Pectinate pedicellariae on the oral intermediate plates, on the 



marginal plates, and also on the dorsal side 



Pectinaster Perrier 

 Pedicellariae, when present, two -jawed and occurring only on 

 the adambulacral and upper marginal plates 



1. Pontaster Sladen 



1. Pontaster Sladen. 



Disk rather small, arms long, pointed ; dorsal side with 

 paxillse. Marginal plates small, each with a large spine. No odd 

 interradial marginal plate. Adambulacral plates with a comb of 

 fine furrow spines and 1-3 spines outside these. Pedicellariae, 

 when present, two-jawed, mainly on the adambulacral plates, but 

 sometimes also on upper marginal plates. 



Only one species, P. tenuispinus (Diib. and Kor.), known to 

 occur* in British seas, but another s^^ecies, Pontaster venustus Sladen, 

 known from the Bay of Biscay and between the Azores and 

 Madeira (1165-4000 m.), may be expected to occur also in British 

 seas. It is distinguished from P. tenuispinus by having only 

 one larger spine on the adambulacrals outside the furrow spines ; 



