1896.] OF THE PALMOZOIC OPHIUROIDEA. 1033 
This new species differs from Protaster forbesi, Hall [No. 6. 
pp. 293-294, pl. ix. figs. 5, 6], by the interbrachial margins of the 
disc being concave : in P. forbesi, moreover, the adambulacral ossicles 
are free distally, and each bears a single large spine: the ambu- 
lacral ossicles of the two species are also differently shaped. 
Fig. 2. 
Protaster biforis: a, outline of the disc and arms; 4, 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 lodg- 
ment of the ventral muscles which moved the arms. 
Genus 2. BuNDENBACHIA, Stiirtz, 1886 [15. p. 83}. 
Diagnosis.—Dise soft and delicate ; covering-plates apparently 
small and thin. Ambulacral ossicles with a dumbbell-shaped body 
and thin tapering wing. The body of the ossicle is apparently 
divided into two pieces by a transverse depression. The adambu- 
lacral plates are small and narrow, and support a triangular spine- 
bearing plate. 
The syngnaths are curved, narrow bars. 
