ASTEROIDEA 55 



4275 m.) ; D. Agassizi Perrier (Bay of Biscay, Azores, 3120- 

 4360 m.) ; D. intermedins Perrier (Azores, 2870 m.) ; D. rigidus 

 Perrier (Bay of Biscay, Azores, 4443-5005 m.) ; D. {Crenaster) 

 mollis Perrier (Azores, 2995 m.) ; D. (Crenaster) semispinosus 

 Perrier (44° N. 15° W., 5005 m.) ; D. (Crenaster) spinulosus 

 Perrier (44° N. 19° W., 4255 m.). For descriptions and figures 

 of all these species reference must be made to the works of 

 Sladen (Asteroidea, '" Challenger "'), Koehler (Monaco, Fasc. 

 xxxiv.), and Perrier (Echinodermes du " Travailleur'' et du 

 " Talisman "). 



Key to the genera of Astropectinidce known from or likely to 

 occur in the British seas. 



1. Upper marginal plates much smaller than the lower ones ; both 



covered with small grains, without any larger spines. Small 

 form ...... 6. Lepty chaster Smith 



Vpper and lower marginal plates of about equal size ; more or 

 less conspicuous spines on lower, often also on vipper marginal 

 plates ......... 2 



2. Oral interradial area very large, with 6-7 series of plates ; lower 



marginals with a series of large spines, upper marginals without 

 any. Madreporite large, naked. Large form 



Tethyaster Sladen 



Oral interradial area very small, or of medium size, with at most 



3-4 series of plates ....... 3 



3. Oral interradial area very small, interradial plates not continuing 



beyond base of arm ; adambulacral plates therefore joining 

 lower marginals in almost the whole length of arm ; spines on 

 lower marginals forming a prominent fringe all round body 

 and arms . . . . . 1. Astropecten Linck 



Oral interradial area of medium size, the inner (adradial) series 

 of interradial plates generally extending to about the middle 

 of arm, separating adambulacrals from lower marginals. 

 Spines of lower marginals not forming a prominent fringe 

 round body and arms ....... 4 



4. Upper marginals thin, confined to side wall of ray ; oral inter- 



radial plates often with peculiar, subvalvular pedicellaria3 

 (Fig. 31). Madreporite very large, covered with paxillas 



Dytaster Sladen 

 Upper marginals not thin and confined to side wall of ray ; no 

 subvalvular pedicellariae on oral interradial area. Madre- 

 porite not very large ....... 5 



5. Madreporite covered with paxillae . 4. Plutonaster Sladen 

 Madreporite not covered with paxilla^ .... 6 



6. Marginal plates distinctly tumid ; side of arm not very high. 



Dorsal side of arm, between marginals, very narrow 



5. Persephonaster Alcock 



