OCR Text |
Show 82 PROF. F. J. BELL ON THE [Feb. 19, outer is thicker than the inner. The ventral plates each beat* a spine ; the ambulacral spinulation is triplacanthid. R = 2 0 0 millim. Hab. Sumatra. OREASTER GRACILIS. Oreaster gracilis, Liitken, Vidensk. Medd. 1871, p. 260. As there is in the British Museum collection a specimen the spread of which must exceed the largest of Dr. Liitken's specimens by more than 50 millim., it may be convenient to give an account of it. R = 2*87r. Disk not elevated; arms narrow at their base, slender in proportion to the disk ; lophial line well marked, but not projecting; the apical spines of moderate height, and a smaller central spine. About 30 superomarginal and inferomarginal plates ; the greater number of these are large, and both sets take part in forming the sides of the arm. In the angles of the arm they are elongated from above downwards ; further out the upper are longest in the direction of the long axis of the arm, while tbe lower are squarish. In the specimen under description the two inferomarginals in the angle of the arm are always, and the corresponding superomarginals are sometimes, provided with bluntly conical spinous projections ; inconspicuous tubercular projections are developed on a few of the quite distal superomarginal plates. Adambulacral spinulation triplacanthid ; in the innermost row ordinarily nine rather delicate spines, of which the median are the longest: the middle and outer rows have generally two spines each ; these are stouter than the inner spines, and those of the median are a little stronger than those of the outer row. The ventral plates are quite distinct from one another, the covering granulation being so arranged that each ossicle seems to have its proper investment; on the actinal surface of the disk these ossicles seem to have no definite arrangement; along the greater part of the arm there runs but a single row of ossicles between the ambulacrum and the inferomarginal plates ; these ventral ossicles are all of the same size, and the larger may be often seen to have pushed their way into the slight space between two succeeding inferomarginal plates. Fair-sized pedicellariae are developed on some of the ossicles that lie nearest to the adambulacral plates. The marginal plates are very regularly granulated, and appear to be altogether devoid of pedicellariae. The granulation on the upper surface is still more delicate; the general appearance of this aspect of the disk is well stated in the words of Liitken, " Dorsum disci regulariter reticulatum, areis poriferis trigonis, nodis trabecularum tuberculiferis ; " but of this specimen it is hardly correct to add "tuberculis minutis." In the face of the fact that this is a larger specimen than either examined by Dr. Liitken, and that the tubercles at the nodal points of the reticulating ossicles are, so far as one can judge, better developed than in his specimens, the question arises as to the extent |