OPIIIOTIIRIX ORSTEDII. I55 



hard to distinguish their true outlines ; they are nearly oval, with a 

 slight peak towards the mouth ; length to breadth as .G : 1 or .8 : 1. 

 Madreporic mouth-shield longer, larger, and more swelled than the 

 others ; length to breadth as 1 : 1. Side mouth-shields soldered closely 

 with surrounding parts, slender and wedge-shaped, their broadest end 

 being toward the arm ; they run along the inner sides of the mouth- 

 shield, and nearly meet at its inner points ; length to breadth, .0 : .4, or 

 .8 : .4 ; as the madreporic mouth-shield extends inwards, the side mouth- 

 shields are here reduced to small triangular })ieces on each side. Gen- 

 eral form of under arm-plates square, but broader than long ; the first, 

 second, third, and fourth somewhat compressed sideways, and varying a 

 little in their proportions ; the rest of the plates, even near the tip of 

 the arm, nearly imiform in shape, each separated a little from its neigh- 

 bors, the outer side a slightly re-entering curve, the lateral sides curved 

 a little, the inner side not well defined, but nearly straight ; length to 

 breadth (ninth), .7:1; about two thirds the length of arm, .0 : .6. 

 Upper arm-plates covered, like those on the sides, with skin ; their outr 

 lines can only be seen in dried specimens ; there are four sides, of 

 which the outer is curved, the two lateral straight, and the inner 

 straight, but so short that the plate looks like a short wedge, with the 

 point towards the disk ; the first three or four plates vary in size, but 

 are all small, the innermost often so small that the side arm-plates 

 nearly or quite meet above ; the plates beyond larger ; length to 

 breadth as follows : second plate, .6 : .8, or .4 : .6 ; seventh, .8 : ,8 ; inner 

 side of seventh plate, A"""^- ; the outer edge of each is slightly higher 

 than the next plate, but does not overlap it. Disk, above, the large 

 radial shields are nearly or quite naked ; the rest of the back takes the 

 form of a ten-rayed star, with a small centre ; the brachial rays are 

 nari'ow, and separate each radial shield from its mate ; the interbrachial 

 rays are wider, and separate the pairs of radial shields ; both are beset 

 with spines ; the measures are as follows : diameter of centre of star, 

 2.4°""- ; length of brachial ray, o""""- ; breadth of same, .4"""- ; breadth of 

 interbrachial ray close to centre, l""™', or 1.2'""'' : width of radial shield 

 at outer edge, 1.4"'°- to l.S-""- ; length of the same, 2'""- to 2.2""™- ; the 

 brachial rays are of even width till near the base of the arm, where 

 they contract ; each bears a single row of about five long, tapering 

 spines ; the interbrachial rays are of even width till near the edge of 

 the disk, where the}' grow wider, and pass over the edge to the under 

 surface of the disk ; the spines long and tapering, irregularly placed in 

 three longitudinal rows, each of seven or eight ; radial shields quite 

 smooth, sometimes with a single spine. The spines of the brachial rays 

 have a length of 1.2''^'"- to l.G""" ; those of the interbrachial rays, l"""- to 

 1.2""°-. The second joint bears only two small, nearly equal, slender 

 spines, having a length to that of the under arm-plate as .8 : .8. Third 



