﻿OPIIIOCOMA RTISKI. 77 



tarj ; for more than half the length of arm, the proportions do not 

 alter much, though the plates 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 Avide and short ; length to breadth, .8 : 1.4 ; still farther 

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

 to breadth, .6 : .8. Disk, above, evenly and rather closely granulated 

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

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

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

 covered Avith fine, obscurely marked scales, about .3""°' long. Arm- 

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

 1.6, 1.6 : 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, .6, .6 : .G. 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 upper spine is generally much 

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

 as many as four spines, Avhile the remaining two or three are nearly 

 equal. The characteristic upper spine is slender, pretty even, nearly 

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

 on first jjair 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 broAvn (Cologne 

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

 lighter ; under arm-plates uniform light-brown ; cheAving ajiparatus and 

 mouth-shields still lighter ; upper arm-plates broAvn, Avith occasionally 

 some much darker, thus making cross stripes. Uppermost arm-spines 

 mottled Avith lighter and darker brown ; loAver spines like under arm- 

 plates ; tentacle-scales like under arm-jilates. 



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

 so much so as 0. echinata, which lives side by side with it. The disk 

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

 longest upper arm-spines lO"""". The color in full-groAvn specimens is 

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

 reddish or to umber broAvn. Small specimens often have the back of 

 the disk ornamented Avith a star, of dark brown, made up of tAvo 

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

 brachial spaces beloAv may be more or less perfect. 



O. Riisei may be distinguished from 0. echinata by the different 

 shapes of mouth-shields and upper arm-spines ; and from 0. oithlo^js by 

 much narrower upper arm-plates. 



