274 THE VOYAGE OF H.M.S. CHALLENGER. 
roundly truncate tip ; their base-line forms a slight curve, trending rather obliquely adorally. 
At a short distance behind the furrow series are three or four low, prismatic, laterally 
elongate granules, which form a slightly arched or straight series, traversing the plate 
slightly obliquely, the aboral end of the series being nearest the marginal or furrow series 
of spinelets. The remainder of the plate external to these is covered with large, low, sub- 
prismatic granules, which may form two or three subregular parallel lines but seldom defi- 
nitely regular. Normally each adambulacral plate bears a single large two-valved excavate 
pedicellaria which is usually placed at the adoral extremity of the first series of granules 
behind the furrow series, although occasionally it is found immediately behind this second 
series, but always adjacent to the adoral margin of the plate. Rarely near the mouth these 
pedicellarize may be rather irregular in construction, and formed of three or more valves. 
The mouth-plates are elongate and triangular, slightly truncate exteriorly, and with 
the free margin of each plate forming a straight line in continuation of the series of adam- 
bulacral plates, the united pair completing exactly the apex of the rectilineal angle of the 
actinal interradial area, bounded by the two adjacent furrows. The actinal surface of the 
plates is plane or very faintly convex. The armature of each plate consists of a mar- 
ginal series of nine or ten short, prismatic, roundly truncate spinelets, exactly similar to 
those upon the adambulacral plates, but which increase slightly in size as they approach 
the inner extremity of the mouth-plate. On the actinal surface of the plate a row of about 
seven or eight large, low, irregular-shaped, prismatic granules runs parallel to the median 
suture. The innermost two of this series might also be reckoned as parallel to the mar- 
ginal series, and three or four similar granules continue a line in this direction. Two or 
three small prismatic granules occupy the angular area between the two main series above 
described, and along the base-line of this area, which abuts on the first free adambulacral 
plate, is a straight series of similar granules. 
The actinal interradial areas are paved with a great number of small, normally quadran- 
gular, but occasionally polygonal, intermediate plates, which fit close together and form a 
compact pavement. The largest are adjacent to the adambulacral plates, and these as well 
as the next one or two series are a little broader than long, the breadth of each row 
diminishing as it recedes from the furrow. The remaining intermediate plates have 
the length and breadth approximately equal, or they may be irregular and trapezoid in 
shape. All the plates diminish in size as they approach the margin, the plates at the 
extreme edge of the area adjacent to the infero-marginal plates being very small. The 
surface of the plates is covered with small, low, uniform granules, which are arranged in 
straight series along the margins of the plates, but show no definite order within this 
boundary. In the row adjacent to the adambulacral plates nearly every intermediate plate 
bears a small excavate pedicellaria with two broad, low, truncate, lamelliform valves, a 
little broader than high. Occasionally two pedicellarize are present. Similar pedicellarize 
are also present on a number of plates in the neighbourhood of the median interradial line 
