1896.] OP THE PAtaiOZOIO OPHI0EOIDBA. 1033 



This new species differs from ProUcster forhesi, Hall [No. 6. 

 pp. 293-294, pi. ix. figs. 5, 6], by the iiiterbrachial margins of the 

 disc beiug concave : in P. forhesi, moreover, the adambulacral ossicles 

 are free distally, and each bears a single large spine : the ambu- 

 iacral ossicles of the two species are also differently shaped. 



Fig. 2. 



I I 



I'rolastcr liijoris : a, oullino of tUc disc and arms ; b, a pair of s^'ngiiatbs. 



Protaster hiforis : diagram of the arm-structure. 



It was suggested in the description of P. sedyiulcld 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. Bundenbaciha, Stiirtz, 188G [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. 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. 



