26 ECHINODERMS OF THE BRITISH ISLES 



This large family is subdivided into several subfamilies. Seven 

 genera are represented in British seas. One more, Thaumato- 

 metra A. H. Clark, is likely to be found there also, viz. the species 

 Th. septentrional is A. H. Clark, hitherto known only from S.W. 

 of Iceland, 2075 metres.^ This genus is therefore included in 

 the key, but not otherwise mentioned in the text. 



Key to the suhjamilies and genera of the family Antedonidce known 

 from or likely to he represented in British seas. 



1. Cirrus - sockets arranged in columns on a conical or columnar, 



usually large, centro -dorsal. Subfam. Zenometrince . . 2 



Cirrus-sockets arranged in transverse alternating rows or irregu- 



larl}^ Centro -dorsal hemispherical, discoidal or conical . 3 



2. Pinnule^ usually about three times as long as Pg 



2. Poliometra A. H. CI. 



PinnuleSi_2 both elongated and similar, markedly longer than 



Pg 3. Leptometra A. H. CI. 



3. Pinnule^ composed of numerous (ca. 50) short segments, of which 



at least the first six or seven, usually nearly all, are broader 

 than long, the distal rarely more than twice as long as broad. 

 Subfam. Heliometrince . . . 7. Heliometra A. H. CI. 



Pinnulcj composed for the most part of much elongated segments, 

 though a few of the basal segments may be short ; the distal 

 segments at least twice as long as broad ... 4 



4. Distal cirrus segments entirely without dorsal processes on their 



distal ends (Fig. 12, 2). Cirri usually short, rarely with more 

 than 20, never more than 30, segments. Subfam. Antedonince 



1. Antedon de Frem. 



Distal cirrus segments always with dorsal edge produced in the 



form of a dorsal spine (Fig. 12, 1). Cirri usually long, 12-40 



segments ; very fragile, often broken. Subfam. Bathy- 



metrince ......... 5 



5. All the cirrus segments short, the longest not so much as twice as 



long as the median diameter . . 6. Orthometra A. H. CI. 



Some of the proximal cirrus segments elongated, at least twice 



as long as the median diameter, usually much longer . 6 



6. All the brachials with strongly produced and very spinous 



edges ...... 5. Trichometra A. H. CI. 



distinguishing this family from the other famihes represented in European 

 seas. A much more elaborate diagnosis would be needed for distin- 

 guishing it also from all the non-European families of Comatulids, which 

 would, however, be outside the scope of a book like the present. 



^ A detailed description of this species is found in A. H. Clark's 

 report on the Crinoids of the Danish " Ingolf " Expedition (vol. iv. 5, 

 1923, p. 12). 



