OPIIIOTIIRIX VIOLACEA. 105 



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

 central ones ; lowest Ones smaller, and more rounded and s})ine-like ; 

 ui)j)ei- ones laiger and uiore flattened. Teeth three, thin, delicate, flat- 

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

 and a curve without; length to ))readth, 7:1.2; madreporic shield 

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

 side of the mouth-shield, makiug thus a l)ridge between the inner ends 

 of the genital slits, are quite cons])icuous. 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 little al)Ove 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 Inroad, 

 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, closelj^ beset with short spines 

 w^ithout lateral thorns, each, however, crowned with three or four thorns ; 

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

 stand thickest. Towards centre of disk 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 Ijeset 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 aljout 

 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 : a1)Ove, 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 



