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Show 1896.] OF THE FALiEOZOIC OPHIUROIDEA. 1029 retained Job.. Midler's two orders, as he did also in his latest paper. In 1892 a short and pregnant paper by Prof. P. J. Bell [1] lifted the classification of the Ophiurids on to a different plane. Bell recognized the great importance of the vertebral ossicles and that they are of three main types : (1) the " streptospondyline," where the vertebral ossicles articulate by saddle-shaped surfaces, which do not bear lateral processes or pits; (2) the " zygospondy-line," where lateral processes and pits on the articular surfaces of vertebral ossicles limit the power of movement; (3) the " clad-ophiuroid" (or astrophiuroid), where tbe vertebral ossicles articulate by hourglass-shaped surfaces. Bell therefore proposed to divide the Ophiurids into three groups : (1) the Streptophiurae, for those with streptospondyline ossicles;_ (2) the Cladophiurae, for those with hourglass-shaped articulations; (3) the Zygophhme, for those with zygospondyline ossicles. The definition of these three orders was no doubt a great improvement on any previous arrangement of tbe Ophiurids. There is, however, considerable difficulty in applying this system to the fossil forms, especially in the case of the Streptophiurae. It appears doubtful whether even some recent genera, as Ophlohelus, can be correctly described as having vertebral ossicles articulating by ball-and-socket joints. But this statement certainly cannot be made of many Palaeozoic Ophiurids, which represent a more primitive condition than that of the recent species ; they are indeed so primitive that they cannot be made to enter into any of Bell's orders. The two most striking characters of these Palaeozoic genera are the absence of ventral arm-plates1 and of true vertebral ossicles. The latter are represented by free paired plates, like the ambu-lacral ossicles of Asterids. The ambulacral ossicles are the most importaut plates in the arms of both Asterids and Ophiurids, so that it is a priori probable that they offer a better basis for classification than the external arm-plates. As w e descend from the Zygophiurae, first to the Cladophiurae, and then to the Streptophiurae, we notice a decrease in the complexity and completeness of the vertebral ossicles. It is not therefore surprising, when w e go back to Palaeozoic times, to find Ophiurids with an arm-structure still simpler than anything found in the Streptophiurae. In these early forms the central arm-ossicles occur as a double series of free plates, below which is an open ambulacral groove. Hence the arms appear, at first sight, to be Asterid rather than Ophiurid in arrangement. Hence I propose to found a fourth order of Ophiuroidea to include those without vertebral ossicles, but which have in each arm a double series of free ambulacral plates, which articulate like 1 This character is also found in the genus Ophioteresis of Bell, one of the most primitive of living Ophiurids; it bas, however, vertebral ossicles with streptospondyline articulations. |
