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Show 1896.] OF THE PALiEOZOIC OPHIUROIDEA. 1033 This new species differs from Protaster forbesl, Hall [No. 6. pp. 293-294, pl. ix. figs. 5, 6], by the interbrachial margins of the disc beiug concave : in P. forbesl, moreover, the adambulacral ossicles are free distally, and each bears a single large spine: the ambulacral ossicles of the two species are also differently shaped. Fig. 2. 1 I Protaster biforis: a, outline of the disc and arms; b, a pair of syngnaths. Fig. 3. Protaster biforis: diagram of the arm-structure. It was suggested in the description of P. sedgwicki that the apparent alternation of large and small ambulacral ossicles in that species and some other genera was probably due to a series of depressions across the ossicles. The present species suggests a possible explanation of the nature of those depressions, for they probably had the same function as the deep pits in the ossicles of the new species ; and these, in all probability, were for the lodgment of the ventral muscles which moved the arms. Genus 2. BUNDENBACHIA, Stiirtz, 1886 [15. p. 83]. Diagnosis.-Disc soft and delicate ; covering-plates apparently small and thin. Ambulacral ossicles with a dumbbell-shaped body and thin tapering wing. Tbe body of the ossicle is apparently divided into two pieces by a transverse depression. The adambulacral plates are small and narrow, and support a triangular spine-bearing plate. The syngnaths are curved, narrow bars. |
