184 ECHIN0DERM8 OF THE BRITISH ISLES 



2. Disk with a deep notch in the edge of each interradius . 3 

 Disk without a notch in the interradial edge 



1. Ophiomitrella Verrill 



3. Mouth papillae long, spinifonn ; three spiniform tentacle -scales. 



Disk w^ith strong spines . . , Ophiocamax Lyman 



Mouth papillae broad, scale -like, or papilliform ; one tentacle 



scale .......... 4 



4. Disk with strong spines ; three mouth papillae on each side of 



jaw, the outer very broad . . Ophiothamnus Lyman 



Disk with thorny stumps ; five mouth papillae, the outer not 



distinctl}^ wider than the others . Ophioplinthaca Verrill 



5. Tentacle pores large, with several (5-6) small, spine-like tentacle 



scales ....... Ophiotrema Koehler 



Tentacle pores small, inconspicuous ; only one tentacle scale 



Ophiacantha Miill. and Troschel (partly) 



6. A rather thick skin covers both disk and arms, concealing the 



plates of both ..... Ophiolehes Lyman 

 A rather thick skin covers the disk, leaving only end of radial 



shields free. Arms not covered by skin 2. Ophiotoma Lyman 

 Neither disk nor arms covered by a thick skin ... 7 



7. Mouth papillae numerous, wide, flattened, turned outwards and 



arranged in two or more imbricating rows, covering the whole 

 jaw . . . . . . . Ophiomyces Lyman 



Mouth papillae simple, usually in a single series, not arranged 

 as above ......... 8 



8. Tentacle pores large, conspicuous, with two scales, one large, 



spine-like, at the inner corner and a small one at the outer 

 side edge. Adoral shields with a long spine w^hich covers the 

 second tentacle pore. Disk scales distinct, each wath a long, 

 pointed spine ..... Ophiojnedea Koehler 



Tentacle pores small, inconspicuous, with one or two equal-sized 

 tentacle scales (or, rarely, more) at the inner corner. Adoral 

 shields without spines ....... 9 



9. Dorsal plates of arms each divided into a small, rectangular 



proximal and a larger, rounded distal plate. Radial shields 

 forming elongate crests, but covered by scales, as is the whole 

 dorsal side of the disk, each scale carrying a short spine. 

 Tentacle scale indistinct or none ^ . 4. Ophiochondrus Lym. 

 Dorsal plates of arms undivided 



3. Ophiacantha Miill. and Troschel (partly) 



The species of those genera not yet known from the British 

 seas, which include more than one North Atlantic species, are 

 thus distinguished : 



1 These characters fit the British species Ophiochondrus armatus 

 (Koehler), but not the genus Ophiochondrus in general. In fact it is very 

 doubtful whether the British species is justly referred to this genus. (See 

 below, p. 196, under 0. armatus.) 



