OPHIUROIDEA 157 



the thick skin. Side arm plates small, occupying only the lower 

 side edge of the arms. Spines downwardly directed (or at least 

 partly so), often transformed into hooks or thick, spiny clubs. 

 The arms, which may be simple or branched, are moved mainly 

 in a vertical direction, rolling in or up, or winding themselves 

 round corals or the like, the hourglass-shaped articular surfaces 

 of the vertebrae making such movements possible. Genital slits 

 mostly short, broad, often placed vertically on the edge of the 

 disk at the base of the arms. There may be one madreporite in 

 each interradius. 



Two families of this order are represented in British seas, but 

 one more, the family Asteroschematidce may be expected to be 

 represented there also, two genera of that family being known 

 from the North-east Atlantic, viz. Asteroschema Orsted and 

 Lutken (one species, A. inornatum Koehler, found in the Bay of 

 Biscay and off Madeira, 1480 m.), and Ophiocreas Lyman (one 

 species, 0. oedipus Lyman, found off Madeira, 1500-1968 m.). 

 This family therefore is included in the key. The two genera 

 are very easily distinguished through one, Asteroschema, being 

 covered all over with fine grains, the other, Ophiocreas, by a thick, 

 soft skin. The families Astronychidse and Asteroschematidse are 

 by some authors regarded only as subfamilies of the family 

 Trichasteridce, or even simply included in the latter family. 



Key to the families of Euryalce known from or expected to be 

 represented in the British seas. 



1. Arms branched ; arms covered with grains and bearing rings 



of hooks at least on the outer part . II. Gorgonocephalidce 



Arms unbranched. Arms without rings of hooks . . 2 



2. Disk large, or fairly large ; arms slender, distinctly defined 



against the disk ; three or more arm spines ; only one madre- 

 porite ...... I. Aster onychidce 



Disk very small, arms very stout, often very indistinctly defined 

 against the disk ; only two unequal arm spines ; one madre- 

 porite to each interradius ^ . . . Asteroschematidce 



I. Family Asteronychid^ 



Disk fairly large, well marked off from the long and slender un- 

 branched arms, which are covered by a thick skin. Disk covered 

 with skin or rather distinctly scaled. Genital slits small, just 

 outside the mouth shields, not vertical. Mouth papillae fairly 



^ The madreporites are generally not to be seen without special 

 preparation. 



