328 THE VOYAGE OF H.M.S. CHALLENGER. 



•well produced, flat, tapering to a narrow extremity. Interbrachial arcs very wide and 

 round. Lateral wall or margin angular in the interbrachial arc and at the base of 

 the rays, but becoming vertical on the outer part of the ray, the section of the ray 

 being there quadrangular in consequence. 



The marginal plates are well developed and form a conspicuous border to the 

 abactinal surface. The supero-marginal plates are twenty-four in number from the 

 median interradial line to the extremity. In the interbrachial arc the surface of the 

 plates is only slightly curved, thus forming the bevelled slope above noticed, and the 

 supero-marginal plates do not there attain the extreme margin in consequence of the 

 extension of the infero-marginal plates, which are visible when the starfish is viewed from 

 above, sometimes extending a distance nearly equal to one-third of the breadth of the 

 supero-marginal plates, — the amount seen varying according to the posture of the rays 

 and the relative inflation of the disk. On the outer part of the ray, however, the abacti- 

 nal and lateral areas of the supero-marginal plate are at right angles to one another, with the 

 junction abrupt and subangular. The length of the supero-marginal plates is slightly 

 greater than the breadth throughout the ray, excepting perhaps the innermost two or 

 three in which the dimensions are subequal or with the breadth very slightly in excess. 

 The height of the supero-marginal plates, where they enter vertically into the lateral wall, 

 is rather more than half the length. The surface of the plates is covered with a mode- 

 rately thick membranous tissue, which is continuous over the whole abactinal area of the 

 disk and rays, and is beset with small, uniform, rather widely spaced granules. No spines 

 of any kind are borne on the supero-marginal plates, but towards the extremity of the 

 ray there is a tendency towards the formation of a low tubercular elevation on the angular 

 rounding of the plate near its aboral end. Frequently on the plates that border the disk 

 there is one, or sometimes two or three, small pedicellaria?, having a pair of comparatively 

 elongate pincer-formed jaws, placed in a small circular cavity. 



The infero-marginal plates correspond in number and length to the superior series, 

 and are like them covered with membranous tissue and a similar small granulation. The 

 breadth of the innermost four plates on each side of the median interradial line is rather 

 greater than the length, but beyond this the length is the greater dimension throughout 

 the ray. The bevelled angular edge of the infero-marginal plates which forms the extreme 

 margin of the disk, bears a subregular horizontal series of four or five very short, conical 

 pointed spinelets, sometimes with one or two additional above the line and sometimes 

 with a little irregularity. These marginal dog-tooth-like spines do not extend beyond 

 the area of the disk. ' Along the ray the infero-marginal plates, have a small, poorly 

 developed tubercule on the rounding between the actinal and lateral areas and close 

 to the aboral end of the plate, which becomes more definitely developed towards the 

 extremity of the ray; and is much more distinct throughout than the low incipient 

 tubercule noticed on the outermost supero-marginal plates. 



