KEPORT 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 Tliyonidium and other Holothuroids. These spiuelets 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 Echinoids. 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 arc, 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 arc, 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. 



