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Show 1032 DR. J. W. GREGORY ON THE CLASSIFICATION [Dec. 15, Forbes's figure represents the ambulacral ossicles as being alternately large and small. It is quite possible that each small pair represents a segment, and that the smaller ossicles have been reduced by absorption in order to give space for tbe podia. This explanation, however, seems improbable; for in that case there would be only one podion, instead of a pair, to each segment. The correct explanation appears to be that the smaller pieces are only triangular, distal portions of the ambulacral ossicles, apparently separated from the proximal portion by a groove. The ambulacral plates of Cheiroptaster giganteus, Stiirtz [16. pl.xxx.], for example, are forked, and the junction is depressed; if we only knew this genus from internal casts, it would appear that the two prongs of the forked piece were separate. Many old figures represent Palaeozoic Ophiurids as having the ambulacral ossicles alternately large and small; but it is quite possible that the explanation suggested will account for all such cases. PROTASTER BIFORIS 1, n. sp. (Figs. 2, 3, p. 1033.) Diagnosis.-Disc fairly large; interbrachial outlines concave. The syngnaths2 are simple, prominent, and stout. The ambulacral ossicles consist of a thick body and a stout curved wing. The distal margin of the ossicles is notched by a depression for a ventral muscle-field, which also cuts into the proximal margin of the adjoining ossicle. Owing to these muscular depressions the arm has apparently two series of pores. The adambulacral ossicles are massive and taper slightly to their distal ends; they are closely attached and form a regular series of marginal plates. Arms very flexible. Dimensions: Length of longest arm 18 mm. + x. Diameter of arm at the base 2*5 „ „ arm near the distal end 1 „ ,, mouth 1-5 „ Length of syngnath 1 ,, Width of ambulacral furrow at edge of disc. 1*25 ,, Distribution.-Wenlock Shale. Castell Dinas, Bran, near Llangollen. Mus. Pract. Geol. No. VI. | T 6 T. Affinities.-This species of Protaster differs from P. sedgwicki, Forbes, in having concave interbrachial outlines and roughly triangular adambulacral ossicles, and also by the character of the ambulacral ossicles. Tbe species is of iuterest as showing that the flexibility of tbe arms is clue to the great development of some ventral inter-ambulacral muscles. Thereby the arms could be rolled up ventrally and the ambulacral furrow thus protected. 1 Biforis, having two holes or openings; a character due to tbe appearance of the ventral muscle-pits. 2 " Syngnath," the united piece formed of mcuth-frame and jaw. The "Mund-ecksteck." |
