OCR Text |
Show 1891.] BATHYBIASTER VEXILLIFER. 229 nerve-cord; the ossicles are broad and strong ; the orifices for passage of the tubes are very deeply set, and the walls are so excavated as to form a pit which shelves inwards (Plate XXIV. fig. 2). The adambulacral plates form projecting angles into the groove, and the sides of this angle are at right angles to one another. Fixed on the angle is a short spine and on either side there are generally four, none of which are long, all of which are blunt, and the three inner of which are much broader than the fourth or outermost. Attached to the spine at the angle is a blunt movable spine-like process, the possession of which is the cause of the specific name 1, and which may be, indifferently, spoken of as a " vexillum." The single specimen has, as I have said, been unfortunately dried, and I can say nothing, therefore, as to whether or not there are, as in Bathybiaster pallidus, any elastic peduncle or strong muscular fibres, while the membrane which, apparently, surrounded the spine has shrivelled up in the drying. All that can be said, then, is that the spine has a shallow groove along its upper end, and that its sides are produced into fine denticulations (see Plate XXIV. fig. 1), which recall, though they by no means equal, the denticulation of the pedicellarise of B. pallidus. It is to be hoped that the species, when next dredged, will be so preserved that a satisfactory and detailed account of these spines may be made. The buccal apparatus is grooved, and projects well into the angles of the mouth (Plate XXIV. fig. 5) ; it is closely covered with two rows of about fifteen flattened stout subequal spines; the adambulacral plate on either side of it is elongated and flat, not angulated where it projects into the groove (Plate X X I V . fig. 5). The next distal plate has a long spine-bearing side and a much shorter side with no spines ; the next has three spines on its shorter side, and in the next the proximal and distal sides of the plate are almost subequal, in the next they are equal. The dried ambulacral suckers are only slightly conical. The intermediate plates in the interbrachial angle and along the sides of the ambulacra are densely covered with squamiform granulations (Plate XXIV. fig. 5), which become larger, looser, and more erect near the angles of the mouth and near the sides of the ambulacra. The two rows of marginal plates (supero-and infero-marginal) are so closely approximated and covered with so uniform a granulation, that it is almost possible to believe that there is a single and not a double row of plates. They are strictly confined to the sides of the arms, which they form alone; the fact that the infero-marginal plates practically take no part in forming the lower surface of the arm may be explained by the flattening out of the ambulacral 1 Described by Wyville Thomson in the following terms:-"The inner spine of each comb on the side of the ambulacral groove is longerthan the others, and bears on the end a little oblong calcareous plate usually hanging from it somewhat obliquely like a flag, with sometimes a rudiment of a second attached to it in a gelatinous sheath, which makes it probable that it is an abortive pedicel laria." |