324 A. E. Verrill — JVbrth American 02)hh<roidea. 



* 0. fraterna Ver. When full grown the arm-spines usually 

 closely approximate dorsally (see p. 321). 



* 0. ahyssicola Sars. Spines six, short, nearly smooth. Disk cov- 

 ered with fine ci'otchets. 



* O. anomala Sars. Six arms. Oral shields narrow, or acute. 

 Disk-spinules short, thick, conical or obtuse, roughly serrulate or 

 thorny. 



ff. — The distal oral papillge, or oral tentacle-scales, are clustered 

 or form a double row ; all spiniform. Tentacle-scale spini- 

 form. 



* 0. enopla Ver. Arm-spines seven or eight, roughly serrulate. 

 Disk covered with small, short, obtuse stumps, having several termi- 

 nal thorns. 



ee. — Basal rows of spines not very closely approximate dorsally. 

 Oral papillsB in a simple row. 



* 0. fraterna Ver. Disk covered with very small thorny spinules 

 and crotchets and some rough granules. Oral papillae three, acute, 

 spiniform. Arm-spines eight, serrulate. Tentacle-scale small, flat, 

 subacute. Arm-spines, in large specimens, are approximate dorsally. 



O. cosmica Lym. Disk with coarse thorny stumps having several 

 points at the end. Oral papillae three, stout, conical. Tentacle 

 pores large, with one large scale. Upper arm-plates slightly joined 

 at base of arras. 



dd. — Arm-spines decidedlj^ thorny or prickly, and usually glassy, 

 mostly long and slender. 

 g. — Basal rows of spines approximate dorsally. Side arm-plates 

 very prominent. Disk with small, slender crotchets or 

 branched spinules. 



0. aspera Lym. Arm-spines nine or ten, slender, very thorny. 

 Disk covered with fine thorny crotchets and stumps, terminated by 

 two to six points. Tentacle scale single^ flat, larger at the end, and 

 thorny or lobed. 



*0. milles^nna Ver. Arm-spines ten, long, roughly sex'rulate. 

 Disk closely covered with small, thorny or branched spinules. 



0. 2)entac7'inus Ltk. Arm-spines six, upper ones very slender, not 

 very thorny. Disk with fine crotchets. Tentacle-scale single, small, 

 flat. Distal oral papilla flat. 



