264 



Mr. E. Billings on the Structure of 



width of twenty-two lines ; and the ten together would form a 

 riband about eighteen inches in length and nearly two inches 

 wide. The object of the folding is, of course, to confine this 

 large amount of sm-face to a small space — an arrangement 

 Avhich at once proves the function to be respiratory. Of those 

 figured by Mr. Rofe, P. eUij^ticus^ Sowerby, appears to have 

 only one fold ; P. inflatus, idem, 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. florealts, Say, has five folds situated at a distance from the 

 inner surface of the lancet plate, as in P. ohesus. From the 

 form of the organ, I think that Mr. Rofe's specimen cannot be 

 the species called P. Jiorealis by Say. 



If it be granted that these organs are respiratory in their 

 function, then their five apertures should be called sj^tiracles, 



Fig. 12. 



Fiff. 13. 



Fis. 14. 



Fig. 15. 





Fig. 12. Diagrams of one pair of the liydrospires of a Pentremite : a, the 

 inner side ; b, the outer, or side attached to the shell ; /, the fissures. 

 Fig. l."3. Section across an ambulacrum of a specimen of P. Godoiii, en- 

 larged .3 diameters : /, lancet plate ; f/, amhulacral groove : 2) p, pores 

 leading into the liydrospires ; h h, the two hvdrospires, in transverse 

 section. Fig. 14. Ideal figure of a transverse section through an entire 

 specimen, showing the ten hydrospires : /, one of the five lancet plates ; 

 p p, pores ; r r, the two branches of one of the radial plates. Fig. lo. 

 Summit of P. conoicleus : a, anterior side ; g, ambulacral grooves (copied 

 from Dr. Shumard, but with the ovarian pores added). 



