166 THE VOYAGE OF H.M.S. CHALLENGER. 
Seven cribriform organs are present in each interbrachial are; they are very widely 
expanded, and cover nearly the whole of the plates, adjacent organs almost touching 
in the neighbourhood of the horizontal suture. Their structure is papilliform. (See 
Pl XX Ui hone): 
The ambulacral furrows are narrow, and when they are contracted and closed in by 
the spinelets the tube-feet are entirely hidden from view. ‘The adambulacral plates are 
elongate and subrhomboid in form, and present an angular prominence towards the 
furrow, the adoral side of the angle being much shorter than the aboral; from this 
circumstance results a singularly elegant festooned appearance, when the furrow is viewed 
as a whole. The armature of the adambulacral plates consists of four spines, except 
close to the mouth, three being placed on the aboral facet of the furrow margin of the 
plate, and one on the short adoral facet. The three innermost plates of the furrow 
immediately succeeding to the mouth-plates have five or six spines, and the angular 
prominence into the furrow is less pronounced. The spines of the adambulacral armature 
are uniform in size and shape throughout the ray; they are small, short, flat, and 
terminated abruptly with a lanceolate point, and all are in connection at their base 
with the common investing membrane of the plate. On the actinal surface of the plate, 
behind the furrow series of spines, are three to five small granules, arranged in a slightly 
curved line, which appears to follow the rounded margin of the adoral extremity and 
the outer side of the adambulacral plate. The most adoral of these granules show a 
tendency to develop the flat and pointed form of the furrow series, and there is little 
doubt that they are the representatives of an actinal or secondary series. 
The mouth-plates are large and prominent, each curving down in a coulter-like form 
until the margins which fall in the median line are at right angles to the plane of the 
actinal surface. The prominent median keel thus produced slopes with a regular curve 
adorally and aborally, its longitudinal profile being almost semicircular. The median suture 
is not closed, but rather widely open, and expanding towards the aboral extremity of the 
plates exposes a portion of the odontophore. Each plate bears a short, robust, conical- 
pointed spine at its innermost extremity, the companion spine of the adjoining plate 
standing parallel. ‘There is thus a pair of short, but conspicuous mouth-spines directed 
towards the centre of the actinostome from each mouth-angle. The remaining mouth- 
spines, which are six in number, are uniform in size and shape with the spinelets of the 
adambulacral armature, and are arranged along the margin of the plate which abuts 
on the furrow. A few prominent granules are present on the surface of the mouth- 
plates, but do not in the specimen under description appear to be arranged in any 
definite order. 
The actinal interradial areas are large and extensive, flat, and covered with oblong 
squamiform plates. On the outer portion of the area these plates are about twice as 
broad as long, and are arranged in columns parallel to the median interradial line. 
