82 OPHIOCOMA ECHINATA. 



times only two, or again four ; and in addition there is sometimes 

 another very small papilla below and outside this row. Teeth four, 

 their inner ends nearly square, upper one more tapering and rounded. 

 Mouth-shields a little longer than broad, squarish, with rounded corners; 

 len^j-th to breadth, 2.6 : 2. Side mouth-shields small, like elono-ated tri- 

 angles, lying close to, and soldered with, the sides of the mouth-shields, 

 their inner ends nearly on a line with those of the mouth-shields. The 

 general shape of the under arm-plates at the base of the arm, and so 

 outwards nearly to the tip, is square ; lengths to breadths as follows : 

 second j^hite, 1.4 : 1.4 ; sixth plate, 1.4 : 1.8 ; about two thirds the length 

 of the arm, 1,2 : 1.6 ; plates on the tip joint, .8 : .6. The plates have 

 really six sides, of which the inner and outer ones are parallel to each 

 other, and are straight ; the inner lateral sides are slightly re-entering 

 curves, the outer lateral sides are straight and short. In the sixth 

 plate the proportions of the sides are as follows : inner, 1.6 ; outer, 1 ; 

 inner lateral, 1 ; outer lateral, .6. The rest of the j)lates for some dis- 

 tance along the arm do not materially differ from the sixth plate in 

 their j^i'oportions. The first under arm-plate bounds the outer corner 

 of the mouth-slit, and is very small ; it is somewhat heart-shaped, the 

 length to breadth as 1 : .8. Side arm-plates thick and stout, covered up 

 by a thick skin, except a triangular piece, which fills the space between 

 the base of the upper arm-spine, and the angle formed by the corners 

 of the nearest pair of upper arm-plates. Towards the tip of the arm 

 the side plates are better seen, having a thinner skin over them ; also, 

 as is usual, they encroach more and more on the upper and lower 

 plates ; at the tip of the arm they meet above and below. Upper 

 arm-plates vary somewhat in size and in their outlines ; the general 

 form is oval, with pointed ends ; each overlaps a little the next outer 

 one, less, however, at the tip of the arm, than nearer its base ; the 

 innermost plate is rudimentary ; length of the sixth plate to breadth 

 as 1.6 : 3 ; close to the tip of the arm the plates are heart-shaped, with 

 the point turned inward, the length being to the breadth as .6 : ,8 ; for 

 at least three fourths the length of the arm, though the plates gradually 

 grow smaller, the average proportions of length to breadth among them 

 remain about the same, but single plates vary so as to give such difier- 

 ences as 1.6 : 2,6, 1.6 : 3.8 ; the variable diameter is the breadth. Disk, 

 above, evenly and rather closely granulated, with smooth, bead-like 

 grains, about 25, on the average, to a square mm. ; below, the inter- 

 brachial spaces have a band on each side, running parallel to the geni- 

 tal slits, which is without grains, and shows the under coat of small^ 

 thin, overlapping scales, the largest of them aljout ,1'""' long ; in the 

 midst of the interbrachial spaces the granulation is as above, except 

 near the mouth-shields, where the grains are few and scattered. The 

 genital slits have, near the mouth-shields, a raised granulated ridge 



