136 THE VOYAGE OF H.M.S. CHALLENGER. 
The infero-marginal plates usually correspond both in number and in length to 
the companion superior series, but sometimes a little irregularity occurs at the extremity 
of the ray, and an additional plate may be present. The infero-marginal plates are not 
so high as the superior series, and are longer than high, excepting perhaps one or two 
of the innermost plates in the interbrachial arc. The actinal edge of the plates is curved 
outward, which gives a festooned appearance to the lower margin of the series; and after 
the first two plates in the interbrachial are the succeeding ones have their proximal end 
higher than the distal end, so that the series tapers off gradually towards the extremity 
of the ray. 
One cribriform organ is present in each interbrachial arc. It is situated on the 
median interradial suture, and is rather broad and well defined. The structure is lamelli- 
form, and has been described above. (See Pl. XX. figs. 3 and 4.) 
The ambulacral furrows are wide, open, straight, tapering from the mouth to the 
extremity, and occupy nearly the whole of the under surface of the ray. The adambu- 
lacral plates are rather short and broad, with the adoral and aboral extremities somewhat 
incurved, leaving a small cavity between adjacent plates, which is filled up with mem- 
brane. The armature of each plate consists of two short, sharply-pointed, conical spinelets, 
the adoral one being the longest. The spinelets are placed slightly oblique, and are 
covered with an investing membrane which unites them together at the base and joins 
them up to the neighbouring pairs, concealing to a certain extent their actual size and 
shape. 
The mouth-plates are moderately large and prominent, sharply upturned along the 
line of junction, forming a swollen semi-tubercular keel considerably elevated above the 
general surface of the actinal area. A single mouth-spine is borne at the mouth-angle, 
placed on the line of suture of the united pair and directed upwards; and two smaller, 
compressed, subtriangular mouth-spines stand on the margin of each mouth-plate near 
their aboral extremity. No secondary mouth-spines are present on the surface of the 
mouth-plates. 
The actinostome is wide, occupying fully two-fifths of the actinal area of the disk ; and 
the mouth is situated in the centre of a naked leathery membrane and furnished with 
a well-developed muscular lip. 
The actinal interradial areas form a fairly regular equilateral triangle—the distance 
from the innermost point of the mouth-angle to the place where the area terminates and 
the marginal plates join up immediately to the adambulacral plates being about half-way 
out on the ray, and approximately equal to the base of the area that extends along the 
interbrachial arc. The area is covered with a compact pavement of calcareous plates, 
subhexagonal or subquadrate in form, arranged in columns parallel to the median inter- 
radial line. The whole is covered with a thick membrane, through which the plates are 
scarcely perceptible until the specimen is dried. 
