246 ECHINODERMS OF THE BRITTSH ISLES 



2. Ophiocten Ltitken. 



Disk flat, usually covered with small scales, among which the 

 l^rimaries are more or less conspicuous. Edge of disk usually 

 sharp, forming a distinct limit between dorsal and oral side. 

 Notch above arms feeble, the arms thus not having the appear- 

 ance of issuing from the dorsal side. Usually a continuous comb 

 of papillae across the base of arms. Second tentacle pore outside, 

 not opening into the mouth slit. 



Two species known from the British seas, but two more have 

 been recorded from the N.E. Atlantic, and may be expected to 

 occur also in deeper water off Ireland ; these are : 0. hastatum 

 Lyman (syn. 0. longispinum Koehler), found off the Azores and 

 off the N.W. coast of Spain, 1674-1940 m. {" Challenger ", 

 " Hirondelle ", " Princesse Alice "), widely distributed in the 

 Atlantic and (provided that Koehler is right in regarding 0. 

 pacificum Ltk. and Mrtsn. as a s^oionym of it) also the Pacific, 

 ca. 235-2830 m. ; 0. latens Koehler, 42° 19' N., 23° 26' W., 4060 m. 

 {" Talisman "), Bay of Biscay, 4380-4700 m. ('' Prmcesse Alice ", 

 " Michael Sars "). These species are thus distinguished : 



Key to the species of Ophiocten known from or likely to 

 occur in the British seas. 



1. Mouth shield much longer than broad, reaching nearly to the 



edge of disk ; tentacle scales very large 



2. O. scutatum Koehler 



Mouth shield scarcely longer than broad, or broader than long, 



not reaching half-way to the edge of disk ; tentacle scales 



small .......... 2 



2. Mouth shields distinctly broader than long ; no papillse along 



edge of proximal dorsal plates of arms. (" Talisman " Oph. 



PL XVIII. 11-12) .... O. Za^ens Koehler 



Mouth shields as long as, or a little longer than broad. INIostly 



a comb of papillse along outer edge of proximal dorsal plates 3 



3. Upper arm spine much longer than the 2(3) lower ones, equalHng 



in length \\ arm joints or more. (" Challenger " Oph. PI. IX. 

 10-11; '' Monaco '\ xii. PL VII. 32-33) O. hastatum Lym. 



Upper arm spine not much longer than the lower ones 



1. O. sericeuni (Forbes) 



