620 THE VOYAGE OF H.M.S. CHALLENGER. 
margin of the plate gently concave and forming a little bay along which the ambulacral 
tube-foot passes. Adjacent plates are united by a rather broad tract of muscular 
ligament, and when this is removed by alkali the plates appear to stand widely spaced. 
The armature of the adambulacral plates is very simple. On the middle of the plate one 
large spine stands perpendicularly ; this is more than half the length of the lateral spines 
above described, its measurement being 6°5 mm., and like these it is clothed in a precisely 
similar membranous sheath, crowded with pedicellariz, and furnished with a similar sac- 
cular expansion at the tip. On the inner portion of the ray this spine is truncate at its 
distal extremity and often flaring ; beyond the ovarial region it is tapering and sharply 
pointed, and the spines increase slightly in length till the maximum is attained within the 
middle third of the ray. At the aboral extremity of the plate, and immediately on the 
furrow margin, is a very minute spinelet not more than a millimetre in length, and directed 
at an angle of about 45° over the furrow. This spine is cylindrical, tapering, and covered 
with a closely-fittng membrane, without the crowded pedicellariz ; two to six pedicel- 
larize, however, usually stand in a little group, and form a sort of collarette midway between 
the extremities of the spine. Occasionally this small inner spine is accompanied by a 
companion spine of similar size and character, standing close to it, but rather more adoral 
and higher in the furrow, On the outer margin of the plate, and articulating on a small 
rudimentary infero-marginal plate ankylosed to the adambulacral plate, and appearing 
like a subtubercular elevation near the aboral end, is the long lateral spine above described, 
and this as already noticed is usually present only on alternate plates. No other spines 
are borne on the adambulacral plates. 
The actinostome is large and of wide expanse, occupying three-fifths of the actinal 
surface of the disk, its diameter being 12 mm., in a disk of 20 mm. diameter. Within 
its periphery, and at a higher level within the disk, extends a more or less broad tract of 
buccal membrane, in the centre of which is the large mouth. The muscular system of 
the buccal membrane is strongly developed, and the margin of the mouth is crenulated by 
the numerous plications, the lines of which upon the membrane near the mouth-opening 
have a verrucose appearance, 
The mouth-plates are small and inconspicuous, the united pair forming a shield-shaped 
“mouth-angle,” with a well-rounded margin adorally and subparallel sides. The median 
juncture is invisible on account of the investing membrane. On the adoral margin 
three or four mouth-spines stand on each side of the median line, equidistantly spaced, 
radiating apart, and directed over the buccal membrane; they are very little more than 
1mm, long (scarcely 1°5 mm.), the inner pair being perhaps a trifle longer than the 
others ; the third spine (or the fourth when present) is placed a little further back from 
the margin, on the surface of the plate, than its predecessors, and is succeeded by a large 
secondary spinelet, 3°75 to 4 mm. long, quite on the surface of the plate but still near 
the margin ; this is followed by one, or sometimes a pair of small spinelets, in the same 
