170 OPHIOTHRIX DUMOSA. 



pretty acute angles ; outer side sometimes a little re-enteringly curved ; 

 other sides straight ; length to breadth (twelfth plate), .7 : 1.2. Upper 

 arm-plates diamond-shaped, with inner angle truncated, lateral ones 

 usually acute, and outer one more or less rounded ; on the median line 

 a ridge ; length to breadth (fifth plate), .7 : 1.3. Brachial and interbra- 

 cliial rays of disk closely beset with stout spines, of several sizes, most 

 of them long ; the longest 1.7°""-, the shortest .3'"'°-. There seem to be 

 no such short spines, bearing a crown of slender thorns, and remaining 

 jDcrmanently small, as in 0. sjncidata ; but the smaller spines seem only 

 partly-grown large ones. The largest are stout and round, and have 

 rows of five or six thorns on their sides ; they usually end in three 

 blunt thorns ; the smaller ones usually are very thick at the base, but 

 above the first thorn grow suddenly smaller, and taper to a somewhat 

 blunt, thorny end ; these have rows of three or four thorns on their 

 sides ; the smallest spines are little, stout cylinders, ending in a conical 

 clump of nine or ten short thorns ; rarely this clump has an even top, 

 and not more than six thorns. Radial shields nearly bare, with only a 

 few spines on their inner ends and sides ; outer ends touching each 

 other ; where they overhang the arm there is a short, narrow, raised 

 edge, which is white ; length to breadth, 3.2 : 1.6. Spines on interbra- 

 cliial spaces below more slender and scattered, not extending quite to 

 mouth-shields. Arm-spines rounded and stout at their bases, tapering 

 regularly to a blunt point, but little flattened, ending in a crown of 

 short, blunt thorns ; thirteenth joint, eight spines ; lengths to that of 

 under arm-plate, 3.5, 2.2, 2.5, 2.5, 1.8, 1.5, 1, .G : .7. Upper spine always 

 longest and stoutest, with rows of about twenty-four minute thorns on 

 its sides ; it sometimes has a length of 4.5 ; near base of arm usually 

 about 3.8. Tentacle-scales unusually large, though varying somewhat 

 in size ; instead of being like rudimentary spines, they commonly have 

 the regular oval form of a true scale ; length, .2"'"\ Color, in alcohol : 

 above, disk dull cobalt-blue, approaching lead color ; radial shields 

 lighter, each with three or four large dark spots ; upper arm-plates 

 faint bluish, with a darker spot on each side ; along median line of arm, 

 an indistinct stripe of whitish ; arm-spines dark at tips ; below, inter- 

 brachial spaces similar to upper surface ; mouth-shields and under arm- 

 plates irregularly edged with whitish, and spotted with dark blue. 



Variations. — Another specimen had but few of the longest spines 

 on the disk, most of them being of the second or of the smallest size ; 

 the radial shields had no distinct spots, and the under arm-plates were 

 bright blue, with some bandings of whitish. The side mouth-shields 

 sometimes do not meet within. 



This species might be called a coarse edition of O. spiciilata, from 

 which it is distinguished chiefly by the armature of the disk, which is 



