1869.] 



BILLINGS — STRUCTURE OP CRINOIDS, ETC. 



291 



amount of surface to a small space, an arrangement which at 

 once proves the function to be respiratory. Of those figured by 



Fig. 12. — Diagrams of one pair of the hydrospires of a PentremUe, — a 

 the inner side ; 1) the outer, or side attached to the shell ; / the 

 fissure. 13. Section across an ambulacrum of a specimen of P. 

 Godoni, enlarged 3 diameters. — I lancet plate ; g ambulacral groove ; 

 p p pores leading into the hydrospires; h h the two hydrospires, in 

 transverse section. 14. Ideal figures of a transverse section through 

 an entire specimen, showing the ten hydrospires. — I one of the five 

 lancet plates ; j) j) pores ; r r the two branches of one of the radial 

 plates. 15. Summit of P. conouleus,—a anterior side ; g ambulacral 

 grooves (copied from Dr Shumard, but with the ovarian pores 

 added). 



Mr. Rofe, P. eUipticus Sowerby appears to have only one fold, 

 P. injiatus, id., shows eight folds in one, and eleven in the other 

 hydrospire of the same ambulacrum. Another specimen, figured 

 by Mr. Rofe, under the name of P. florealis Say, has five folds, 

 situated at a distance from the inner surface of the lancet plate, 

 as in P. ohesK'S. From the form of the organ, I think that Mr. 

 Rofe's specimen cannot be the species called P. florealis by Say. 

 If it be granted that these organs are respiratory in their 

 function, then their five apertures should be called spiracles, — 



