CLARK: THE STARFISHES OF THE GENUS HELIASTER. 43 



markedly compressed, among wliich are numerous pedicellariae and papulae. — 

 Actinal surface of disc almost entirely occupied by ambulacra, adambulacral spines, 

 pedicellariae, and papulae; interbrachial areas reduced to a minimum. — Adam- 

 bulacral plates with typically a single, conspicuous, erect spine. ' In young specimens 

 these may all be of equal size, but in adults, near the middle of the ray, larger and 

 smaller spines alternate, so that every other plate has a small spine standing 

 hetween the larger spines of the neighboring plates. The smaller spines are com- 

 monly almost or quite within the furrow. In some specimens the small spines 

 are wholly wanting proximally so that only every other plate carries a spine. As 

 a rule the spines are all of a nearly uniform size near the tip of the ray. On the distal 

 half of the ray, some of the adambulacral plates often carry two spines, one behind 

 the other. Beginning just proximal to the base of the ray and running outward 

 to the tip, a series of large spines is found just outside the adambulacral series, and 

 this is followed by one or two more, each series slightly shorter than its predeces- 

 sor. These additional actinal spines differ greatly in number and size in different 

 specimens, apparently increasing with the age of the animal. The adambulacral 

 spines on the middle and proximal part of the ray are the largest spines of the 

 actinal surface and may be as much as five millimeters long. Along the sides of 

 the ambulacral furrows, among the adambulacral and other spines, are numerous 

 pedicellariae, chiefly of the forficate type and of two quite distinct sizes ( Plate 7, figs. 

 2, 3) ; but the size and abundance of the pedicellariae vary greatly in different 

 individuals. — At the centre of the actinal surface occurs the very large buccal 

 membrane, thin, smooth, and conspicuous, with the moutli at the centre. The 

 membrane in a large specimen (R=150 mm.) is 35 mm. across and the mouth is 

 ten millimeters in diameter. Each oral (adambulacral) plate carries two or three 

 short spines arranged side by side more or less horizontally, the innermost the 

 longest, the others successively shorter. The actinal surface shows more or less 

 of a tendency to become abruptly and deeply concave at the centre, so that the 

 proximal portions of the ambulacra are almost vertical, the adambulacral spines 

 thus lying horizontally and the oral spines vertically. This tendency is much 

 more marked in some specimens than in others ; thus, in a specimen with H=105 

 mm., the buccal membrane is 20 mm above the horizontal portion of the actinal 

 surface of the rays, while in another specimen with R=150 mm. the depression is 

 no deeper ; and in a third specimen with R=48 mm., the vertical distance to the 

 buccal membrane is only five millimeters. As no observations on the living ani- 

 mal have yet been recorded, it is impossible to say whether this buccal depression 

 has any physiological importance or not. It is interesting to note however that in 

 adult specimens where the depression is well marked, the adambulacral spines on 

 its sides are smaller and less prominent, and the pedicels longer and more prominent, 

 than elsewhere on the actinal surface. — Pedicels in a zigzag row on each side of 

 each ambulacrum, scarcely crowded enough to make them quadriserial ; proximally 

 in adults and still more so in the young, they are distinctly biserial. Madre- 

 porite single ; small, slightly convex and irregularly furrowed in young specimens, 

 usually becoming broken up into a number of fragments in adults ; even small 

 specimens may show this fragmentation to some extent. — Color 1 of abactinal 

 surface dark (gray, brown, blackish, or black), rarely more or less variegated with 



1 The color of living Heliasters has never been described ; in all the descrip- 

 tions here given, the colors referred to are those of alcoholic and dried specirhens. 



