ASTEROIDEA 51 



Key to the families of the Paxillosa. 



Cribriform organs^ developed between (some of or all) the marginal 

 plates ; ampullae single . . .1. Porcellanasteridce 



No cribriform organs ; ampullae double .... 2 



Upper marginal plates well developed ; papulae simple ; mostly 

 only a single pair of genital organs at the base of each ray 



II. Astropectinidce 



Upper marginal plates undeveloped (replaced by paxillae) ; 

 papulae bush-shaped. A series of genital organs along each 

 side of the raj-, nearly to the end . . . III. Luidiidce 



I. Family Porcellanasterid^ 2 



Marginal plates very high, but relatively thin and lamellif orm, 

 naked or covered with rather thick membrane. Aboral side with 

 paxillae or single spinelets. Interradial areas on the oral side 

 more or less extensive, paved with squamiform plates, covered 

 with delicate membrane, and sometimes bearing isolated spinelets 

 or fimbriated channels (or "fascioles "). Cribriform organs 

 present. Adambulacral plates large, with a simple marginal 

 armature, uniserially disposed. No intestine or intestinal caeca ; 

 no anus. Tube-feet pointed, ampullae simple. 



Only one genus and species, Ctenodiscus crispatus, has been 

 found in the British seas. There is, how^ever, good reason to 

 expect that the rich fauna of deep-sea Porcellanasteridae known 

 from the region between the Bay of Biscay, the Azores and the 

 Cape Verde Islands will prove to be well represented also in the 

 deeper w^aters off Ireland, and it seems therefore desirable to 

 include these forms in the key. 



Key to the North-East Atlantic genera of Porcellanasteridce. 



1. Cribriform organs between all the marginals ; fimbriated channels 



(fascioles) present in the oral interradial areas 



I. Ctenodiscus Miiller and Troschel 



Cribriform organs localised, not between all the marginals. No 



fimbriated channels on oral interradial areas ... 2 



2. Lower marginal plates rudimentary or absent ; one cribriform 



organ, in the middle in each interradius. Albatrossaster Ludwig 

 Both upper and lower marginals well developed . . . 3 



1 See p. 4t. 



2 Possibly it might be more correct to make the Porcellanasterids a 

 separate suborder, like the Notomyota. 



