298 bulletin: museum of comparative zoology. 



very abundant all over the disk, but as there is much individual 

 diversity and some changes with age, this is not a character of much 

 taxonomic value. Color is seldom distinctive but occasionally some 

 color-marking is very characteristic. Bearing these tendencies to 

 diversity in mind, the various species may be distinguished as fol- 

 lows : — 



Key to the Species of Ophiactis. 



a Upper arm-plates more or less fan-shaped, oval or triangular, the proximal 

 angle being tnmcate to a greater or less degree, when the plates are in 

 contact with each other. 

 b Disk covered by rather coarse, flat, rounded scales between which or on 

 their corners or edges are low, conical tubercles; these increase in length 

 near disk-margin becoming short, sharp spinelets; arms short, about 4 X 

 disk-diameter; oral shields pentagonal widest proidmally . . . symbiota 

 ■h' Disk with or without spines but not as above, 

 c 1 oral papilla on each side of each jaw. 



d Oral shields distinctly wider than long; disk-spines few or none; 

 arm-spines 3 or 4. 



e Adoral plates meet more or less fully interradially plana 



e' Adoral plates do not meet interradially. 

 / Interbrachial areas below without distinct scales . . macrolepidota 

 f Interbrachial areas below covered by large scales . . luteomaculata 

 d' Oral shields not distinctly wider than long, somewhat pentagonal, 

 oval or circular. 

 e Adoral plates meet interradially. 

 / Arms 6, very short, about 2 X disk-diameter; arm-spines 



4 or 5 brachyura 



f Arms longer, 4-8 X disk-diameter. 



g Arms 5; radial shields short, nearly as wide as long; disk- 

 spines numerous nama 



g' Arms 6; radial shields nearly or quite twice as long as wide; 

 disk-spines none or few. 



h Jaws short, their length scarcely twice width of big oral 

 papilla; adoral plates meeting broadly interradially; oral 



shield widest proximally pteropoma 



h' Jaws long, narrow; length more than twice width of 

 slender oral papilla; adoral plates tend to be somewhat 

 parted interradially; oral shield widest distally. .delicata 

 e' Adoral plates do not meet interradially. 

 / Arms 5; disk-spines small but numerous, at least in inter- 

 brachial spaces; arm-spines unspotted ballii 



/' Arms 6; disk-spines few or none. 



