﻿OPIIIOTIIRIX VIOLACEA. 165 



figure, about thirty in number ; those on sides larger and longer than 

 central ones ; lowest ones smaller, and more rounded and spine-like ; 

 upper ones larger and more flattened. Teeth three, thin, delicate, flat- 

 tened, squarish. Mouth-shields diamond oval, with an angle inward 

 and a curve Avithout ; length to breadth, 7 : 1.2 ; madreporic shield 

 larger than others, and rounded. The two plates that join the outer 

 side of the mouth-shield, making thus a bridge between the inner ends 

 of the genital slits, are quite conspicuous. Side mouth-shields triangu- 

 lar, elongated, nearly meeting within mouth-shield proper ; they are 

 closely soldered with surrounding parts, and are hard to see except in a 

 dry specimen. Under arm-plates broader than long, with a strongly 

 re-entering curve on outer side ; inner side nearly straight ; lateral 

 sides curved, or slightly angular ; plates near base of arm have their 

 outer side raised a httle above succeeding plate ; plates within margin 

 of disk narrower, and more closely soldered to each other than those 

 beyond ; length to breadth (fourteenth plate), .7:1; about two thirds 

 out on arm, plates nearly square, a little longer, however, than broad, 

 with a re-entering curve in outer side, though less strong than in the 

 plates near base of arm. Side arm-plates encroaching so much above 

 as nearly to meet each other, even near base of arm. Upper arm-plates 

 short diamond-shaped, with a slight median ridge ; outer and inner 

 angles more or less rounded ; length to breadth (seventh plate), .7 : 1.2 ; 

 about two thirds out on arm, plates have same general form, but are 

 about as long as broad. Disk, above, closely beset with short spines 

 without lateral thorns, each, however, crowned with three or four thorns ; 

 less numerous on radial shields ; about 36 to a sqnare mm., where they 

 stand thickest. Towards centre of di.sk a few longer spines, like those 

 of arms, but more slender and shorter. Below, interbrachial spaces 

 covered with spines somewhat longer than the short ones above. Ra- 

 dial shields beset -with scattered spines ; length to breadth, 2.4 : 1 ; 

 their interbrachial side is strongly curved ; their brachial side nearly 

 straight, or a little re-enteringly curved ; where they meet, just at their 

 outer end, they have a slight protuberance ; for the rest of their length 

 they are separated by a tapering brachial stripe. Arm-spines about 

 eight, long and slender ; largest ones with fifteen to twenty thorns on 

 each edge ; lengths to that of under arm-plate as follows : (seventh 

 joint,) 2.5, 3, 2.7, 2.3, 1.6, 1.6, .5 : .7. Sometimes there are as many as 

 eleven spines, but the additional ones are very short, and are the low- 

 est. About half-way out on arm six spines, lowest one broad and flat, 

 with hooks on its edge ; four upper ones long. Tentacle-scale minute, 

 sharp, conical, usually simple, sometimes with a crown of two or three 

 microscopic thorns. Color, in alcohol : above, bluish purple ; along 

 median line of arm, a narrow stripe of clear white, bounded each side 

 by a dark stripe ; two fine white spots on lateral corners ; arm-spines 



