OPHIUROIDEA 



253 



2. Dorsal plates present, though very small, nearly to end of arm ; 



ventral plates present on both tentacle -bearing joints, but 

 not beyond ........ 3 



No dorsal plates ; a ventral plate present only on the first tentacle - 

 bearing joint . . . . O. africanum Koehler 



3. Arm spines 6-8 ; no conspicuous rosette in centre of disk 



1. O. Lymani Wyv. Thorns. 

 Arm spines 3 ; usually a conspicuous rosette in centre of disk 



O. scalare Lyman 



4. Only the two first dorsal plates contiguous. O. planum Lyman 

 Some 5-6 of the proximal dorsal plates contiguous ^ 



O. armigerum Lyman 



L OpJiiomusium Lymani Wyv. Thomson. (Fig. 138.) 



Dorsal side of disk somewhat elevated ; in the centre a group 

 of small, angular plates, among w^hich the primary plates may 

 be more or less distinct. From the central group radiate ten 



Fig. 138. — Ophioniusium Lymani ; part of oral and dorsal side. x 4. 



series of larger plates, one series in each interradius and one 

 between each pair of radial shields, which latter are large, about 

 half as long as the disk radius. All the larger plates of the 

 dorsal side distinctly tuberculated. Dorsal plates of arms small, 

 triangular, widely separated already from the first free arm- 



^ The two species 0. planum and armigerum would seem to be so closely 

 related as to be hardly distinguishable, unless the difference in the dorsal 

 plates given in the key proves to be constant ; from literature this cannot 

 be seen, and the material of the two species available to the author is not 

 sufficient to judge of the constancy of this character. 



