OPIIIOCOMA RIISEI. 77 



turv ; for more than half the leiif,^th of arm, the proportions do not 

 alter much, though the phites grow smaller, but there are such varia- 

 tions among individual plates as 1.2 : 1.6, 1.2 : 2 ; farther out, plates ill 

 defined, very wide and short; length to breadth, .8 : 1.4 ; still farther 

 out, plates well defined again, heart-shaped, the point inward ; length 

 to l)readth, .(> : .8. Disk, above, evenly and rather clo.sely granulated 

 Mith bead-like grains, about sixteen, on the average, to a square mm.; 

 uudenieath, no granulation, except on a small triangular space which is 

 continued from the upper surface ; the rest of the interbrachial space 

 covered with fine, obscurely marked scales, aljout .o"""- long. Arm- 

 spines on second joint two, their lengths to that of the imder arm-plate, 

 l.(», 1.0 : 1 ; third joint, three spines, 1.4, 1.8, 1.8 : 1.2 ; fifth joint, four 

 spines, 1.8, 1.8, 2, 2.2 : 1.2 ; eleventh joint, four spines, 4.8, 4.4, 4.4, 

 4.4 : 1.2 ; about two thirds the length of the arm, 3.4, 2.8, 2.2, 2.2 : .8 ; 

 close to tip of arm, three spines, .8, .(j, .0 : .6. From near the disk to 

 about two thirds the length of the arm, there are sometimes four, some- 

 times three spines to each joint, these numbers often alternating ; on 

 the joints not enclosed by the disk, the upjier spine is generally much 

 the longest, and is more rounded, particularly on those joints that have 

 as many as four sjDines, while the remaining two or three are nearly 

 equal. The characteristic upper sj^ine is slender, pretty even, nearly 

 cvlindrical, with a thickness to length as .8 : 4.8. Tentacle-scales, two 

 on first pair of pores, and rarely on second pair ; on all the rest, only 

 one ; in shape regular oval ; length of those on third joint, to length of 

 under arm-plate, .6 : 1.2. Color, in alcohol : roof of disk l)rown (Cologne 

 earth), with obscure radiating bands of darker; interbrachial spaces 

 lighter ; under arm-plates uniform light-brown ; chewing apparatus and 

 mouth-shields still lighter ; upper arm-plates brown, with occasionally 

 some much darker, thus making cross stripes. Uppermost arm-sj^ines 

 mottled with lighter and darker brown ; lower spines like under arm- 

 plates ; tentacle-scales like under arm-plates. 



Variations. — This species is common in the West Indies, though not 

 so much so as O. echinafa, which lives side by side with it. The disk 

 often attains a size of 24'"'"-, with arms five or six times as long, and the 

 longest upper arm-spines 1U""°'. The color in full-grown specimens is 

 singularly invariable ; the arm-spines may be more or less inclined to 

 reddish or to und^er brown. Small specimens often have the back of 

 the disk ornamented with a star, of dark brown, made up of two 

 radiating lines in each brachial space. The granulation of the inter- 

 brachial spaces below may be more or less perfect. 



O. Bilsei may be distinguished from O. e.chinata by the different 

 shapes of mouth-shields and upper arm-spines; and from 0. afhlojts ]>y 

 much narrower up})er arm-plates. 



