378 15ULLETIN 76, UNITED STATES NATIONAL MUSEUM. 



Adambulacral furrows fairly wide; tube feet in two rows, with small disks or 

 buttons at the tip, except near the tip of ray where the feet are pointed. Armature 

 consists of typically two (but some plates with tlu-ee) slender furrow spinelets in a 

 longitudinal scries, the adoral slightly the longer (4. .5 mm.) and about as long as the 

 distance between the base of two adoral spinelets of two plates. The spinelets have 

 a flange of tissue broadening distally and continued beyond the tip for a short dis- 

 tance. Aperture papilla situated just back of the aboral furrow spinelet, or the 

 interval between the two spinelets. It is very broad, ovate or obovate, and with 

 tlie fleshy part is about as long as the calcareous portion of the longer furrow spine- 

 let. The calcareous support of the papilla is variable, but is either ovate with a blunt 

 or truncate tip, or is even broader than long with a truncate ragged end. A fleshy 

 tip often extends into the aperture, plugging it up, but is not superficially apparent. 

 It is really a short sacculus and varies in size, apparently never long. 



Free margin of each mouth plate nearly as long as interradial diameter of plate, 

 the combined plates, therefore, presenting a broad margin to actinostome. Marginal 

 spinelets slender, subequal, five or si.\, rarely seven, to each plate, similar to the 

 adambulacral spinelets, and nearly evenly spaced. Suboral spinelets two, some- 

 what thicker and a trifle shorter (about one-half length of interradial diameter of 

 plate). The inner one sometimes moves to the margin and becomes the innermost 

 marginal spine. When there are two suborals there are four or five marginals. 



Actinolateral spines gradually increasing in length and thickness along ray, the 

 twelfth to eighteenth (about) being longest and heaviest. None of the spines quite 

 meet, across the interradial space, those of adjacent ray. From about the seven- 

 teenth or eighteenth the spines become rapidly shorter and the tips end at the free 

 border of the web instead of being embedded in the web. Most of the larger spines 

 are broken (some apparently before the death of the animal), forming a sort of joint. 

 Just above the articulating facet the base of each spine has an abrupt expansion on 

 either side. The membrane is thin, so that the spines are conspicuous. The inter- 

 radial portion between the tips of two series of spines is muscular. 



Color in alcohol, bleached j-eUowish. 



Variations. — The outward appearance of all the specimens is essentially the 

 same. Those from very deep water (station 3307) apparently have a more translucent 

 membrane, but this is in a large measure due to the epidermis having sloughed off 

 either from long immersion in very weak alcohol or from scouring of mud and fine 

 sand in the dredge. In such examples the muscle fibers are much more conspicuous 

 than in normal specimens. However, the supradorsal membrane is weaker in deep- 

 water specimens. There is variation in the number of muscle fibers. Mention has 

 already been made that a minority of the adambulacral plates may have three 

 spines; the number of such plates is higldy variable, and they occur in both northern 

 and southern and in the shallower and deeper water specimens. 



The contour of the disk varies. vSometimcs the long actinolateral spines cause 

 the interradial web to extend outward, forming an interradial corner so that the 

 creature apparently has ten angles and ten sides instead of five. 



Avery remarkable specimen from station 2859 off Prince of Wales Island, ^Uaska, 

 1,5G9 fathoms, was taken along with II. quadrispinosus. It exliibits all the charac- 

 ters of perissovotus except that the adambulacral i)lates have an oblique transverse 



