﻿OPIIIOTIIRIX ANGULATA. 163 



.(1 : .4 ; close to tip of arm, plates longer in proportion. Along the mid- 

 dle of each plate runs a ridge, and there is therefore a continuous ridge 

 Iroiu the base to the point of the arm. Disk, above, with a raised star 

 of tvn rays and a small centre ; interbrachial rays high and broad, with 

 a width of 1.2""' near centre of disk, and of l.G"""' at its edge ; closely 

 beset with very small, short spines, ending in a triple fork ; about 34 

 spines to a square mm. ; interbrachial rays, meeting in middle of disk, 

 make a centre to the star, having a diameter of 2"" ; brachial rays very 

 low and narrow, and tapering to a point before reaching edge of disk ; 

 greatest width, .2°"'- ; each bears a single row of forked spines. Radial 

 shields, length l.e"""-, greatest width 1""°- ; each has about twelve scat- 

 tered, forked spines. Besides short, forked spines, there are on the 

 back of the disk a few longer, thorny spines, having a length of .8""", or 

 1'"°'-. Interbrachial spaces below have a triangular patch of close-set 

 spines, which is a continuation of the interbrachial ray from above, 

 this patch is bounded by a stripe of bare skin, .8™"', which runs along 

 the edge of the genital slits to the mouth-shields. Arm-spines, number 

 of spines and lengths compared with those of the under arm-plates : 

 second joint, two spines, .4, .4 : .6 ; third joint, four spines, .4, .4, .6, .6 : .6 ; 

 fourth joint, six spines, .4, .6, .6, .8, .8, .8 : .6 ; fifth joint, eleven spines, 

 1.6, 1.6, 1.6, 1.2, 1.2, .8, .8, .8, .6, .4, .2 : .6 ; sixth joint, ten spines, longest 

 one 2°"' ; seventeenth joint, six spines, 1.4, 2.8, 1.4, .6, .4, .2 : .6 ; three 

 fourths out on arm, five spines, 1.4, 2, .8, .4, .4 :.4 ; the lowest spine has 

 here the form of a heavy knife, bearing two or three hooks on its edge ; 

 these hooks continue inwards to about the fiftieth joint from the tip, 

 wdiere they gradually take on the form of true spines ; at the tip of 

 arm, four spines, the lowest a hook. The longer and stouter spines, 

 near base of arm, have usually a thorny, rather blunt end, and twelve 

 or fifteen thorns on each edge. Tentacle-scales, from base of arm quite 

 to the tip there is a small, stunted spine, with two or three thorns at its 

 end, which answers to the tentacle-scale. Color, in alcohol : general 

 tint, a faded, grayish cobalt blue ; under arm-plates and mouth appa- 

 ratus yellowish-white ; arms banded with darker blue ; interbrachial 

 spaces on back of disk darker than brachial spaces ; a hglit line run- 

 ning along upper side of arm. 



Variations. — The differences in color of this species, when living, 

 are very great. According to colored drawings of about twenty varie- 

 ties, belonging to Professor Agassiz, the disk may be various shades of 

 vermihon, pink, purple, blue, dull green, brown, and yellow ; the radial 

 shields seem always to differ from the rest of the disk ; the ground 

 color of the arms, also, is usually different from that of the disk, and 

 varies quite as much ; the arms are always banded. In alcohol the 

 color has always a faded look, the specimens Ijeing either whitish, or 



