204 



Mr. E. BilliiifTS 071 the Str/ictiire of 



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

 riband about eighteen inches in length and nearly two inches 

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

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

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

 figured by Mr. Ivofe, P. ellipticus, Sowerby, appears to have 

 only one fold ; P. injiatus, idem, shows eight folds in one and 

 eleven in the other hydrospirc of the same ambulacrum. 

 ^Vnother specimen, figured by Mr, Rofe under the name of 

 P.jiorealisj Say, has five folds situated at a distance from the 

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

 form of the organ, 1 think that 3Ir. liofe'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 sjjiracles, 



Fig. 12. 



Fig. 13. 



Fig. 14. 



Fisr. lo. 





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

 inner side ; b, the outer, nr sido attaclied to the sliell ; /, the fissures. 

 Fig. 13. Section across an ambulacrum of a specimen of/', (indimi, en- 

 larged 3 diameters : /, lancet plate ; //, ambulncral groove ; p p, pores 

 leading into the hydrospires; h h, the two hydrospires, in transverse 

 section. Fig. 14. Ideal figure of a transverse section throu^rh an entire 

 specimen, showin<r the ten hydrospires : /, one of the five lancet plates; 

 p p, pores; r r, the two branches of one of the radial plates. Pig. IH. 

 Summit of P. cnuoirhus : n, anterior side ; 7, ambulacral grooves (copied 

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



