REPORT ON THE ASTEROIDEA. 135 
abactinal surface extends along the ray and tapers off with a graceful curve towards the 
extremity, which causes the rays to have a very short appearance when seen in profile. 
The abactinal area is covered with a thick coriaceous integument, the usual meshwork 
skeleton of calcareous plates being altogether wanting. The membrane is indurated with 
a number of minute circular spicules, some of which bear a vertical spinelet, resembling 
the surface spicules of Zhyonidium and other Holothuroids. These spinelets are sparsely 
distributed over the central portion of the abactinal area and along bands that run there- 
from to the interbrachial arc, in the median interradial line. The spinelets are long and 
thin, and, being made up of fine calcareous rods united by short transverse dissepiments, 
present under the microscope a very open structure somewhat resembling the delicate 
hair-like spines of certain irregular Hchinoids. The spinelets are clothed with thick 
investing membrane, which not unfrequently develops a knob at the extremity, and 
gives a club-shaped character to the appendage. A more or less prominent tubular 
epiproctal prolongation is present in the centre of the disk ; in some examples measuring 
between 2 and 3 mm. in length, but shorter in others. It is a subcylindrical tube 
less than a millimetre in diameter, springing directly from the abactinal area, tapering 
very slightly towards the extremity, and is indurated with a close plating of very 
minute spinulate spicules. . 
The marginal plates form a deep conspicuous band, and stand as a perpendicular wall 
in the interbrachial are, bending gently inward above and below. Along the rays the 
supero-marginal series‘arch well over on the abactinal surface, and leave only a very con- 
stricted space along the median line of the ray between the corresponding plates of each side. 
The supero-marginal plates, which are six or seven in number exclusive of the terminal, 
are bounded by straight lines, and vary from a quadrate to a subrhombic form accord- 
ing to position. The height and length are nearly equal, the latter being usually the 
greatest dimension, although in some specimens the proportions may be reversed. Nor- 
mally each of the supero-marginal plates bears a short conical spinelet on its upper 
edge, but not unfrequently these become aborted into little more than tubercles, and are 
sometimes absent altogether, especially on those plates which are innermost in the inter- 
brachial are, and sometimes also on the penultimate plate of the ray. The terminal 
plate is moderately large and prominent, with the abactinal surface slightly tubercular, 
the adoral margin being deeply indented in the median line, and the lateral angles fully 
rounded, The whole plate is directed at a slight angle upward in relation to the ray, 
a circumstance which emphasises the strongly upturned appearance presented by the 
rays, the general habit of which seems always to be more or less bent upward or back- 
ward. Three moderately robust spinelets are borne on the terminal plate—one placed at 
the summit of the tubercular swelling in the median line of the ray and directed verti- 
cally upwards, and two somewhat smaller ones which stand one on each side at the 
extreme anterior lower angles of the plate and directed outward. 
