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Show 1030 DR. J. W. GREGORY ON THE CLASSIFICATION [Dec. lo those of Asterids and of the Echinid Palaeodlscus. As the two elements which have fused to form the vertebral ossicles of later Ophiurids are unattached in the members of this order, I propose for it the name Lysophiurae1. N o one has worked at the Palaeozoic Ophiurids without being impressed by the unsatisfactory nature of' many of the genera. In m y earliest palaeontological paper (1889) I pointed out that Protaster would have to be split up into more than two genera [5. p. 27]. Stiirtz, both in 1890 [16. p. 245] and 1893 [17. p. 19], also insisted that Protaster includes a miscellaneous group of species, and that the Protasters of Porbes, Billings, Hall, and myself are distinct. I shrank from the task of dismembering this genus in 1889, as I hoped for better specimens of the type species. None such, however, have been forthcoming. As I a m now bound to attempt to indicate the relations of the fossil and recent forms in an account of the Ophiuroidea for Prof. Lankester's 'Oxford Natural History,' I delay no longer. In order to simplify m y task in that place, I offer the following synopsis of the classification of the Palaeozoic Ophiurids, with diagnoses of some of the genera. Order I. LTSOPHIUR^. Diagnosis.-Ophiuroidea of which the ambulacral ossicles are alternate and are not united into vertebral ossicles. There are no ventral arm-plates, and the underside of the arm is occupied by an ambulacral furrow. Remarks.-This order includes a group of Palaeozoic Ophiurids in which the arm-structure is on the same plan as in the Asterids; for there are no ventral arm-plates, there is an ambulacral groove, and the ambulacral plates are in double series. The members of the order differ from the Asterids by having the arms sharply marked off from the disc ; while the alimentary canal was, in all probability, entirely limited to the disc. So far as is known at% present, the order was limited to the Palaeozoic period; but it is necessary to consider whether a few recent forms ought not to enter it. In Ophiohelus and Ophiotholla the ambulacral plates occur as pairs of rod-like plates, instead of as vertebral ossicles. They therefore, in this respect, resemble Lysophiurids. On application to the Zoological Department of the British Museum, I find that both genera are represented only by the small single specimens dredged by the ' Challenger.' It is too great a responsibility to subject these fragile type specimens to the risk of re-examination, especially as the nature of the articular surfaces could not be determined without dissection. Both specimens are so small, that, as Prof. Bell suggests, it is quite possible they are not mature. The members of the two genera, however, differ from the Palaeozoic Lysophiurae in three respects : they have the ambulacral From \ti<Tis, dissolution, unattachment. |
