408 On the Structure of the Shell 



sometimes on a diminution, of the body of the molluscous 

 inhabitant. An oyster kept without food will frequently ex- 

 pend its last energies in secreting a new nacreous layer, at a 

 distance from the old internal surface of the concave valve, cor- 

 responding to the diminution of bulk which it has experien- 

 ced during its fast, and thus adapt its inflexible outward case 

 to its shrunken body. 



In the calcareous tube exuded from the elongated mantle 

 of the Septari<e, Lam., the closed extremity of the tube is di^- 

 vided into chambers by a succession of calcareous layers, 

 having a regular concavity towards the open or siphonic ex- 

 tremity of the shell. These concave septa are composed en- 

 tirely of the nacreous constituent of the shell. A similar 

 structure is met with in the genus Vermetus : in the speci- 

 men figured {fig. 20). of Vermetus gigas, they were six in 



Vermetus gigas. 

 number ; they are thin, smooth, and closely resemble the par- 

 titions in the Nautilus and Spirula, save in the absence of 

 the siphonic perforation. 



Among bivalves the Ostrece not unfrequently present shal- 

 low and irregular empty spaces in the substance of the shell ; 

 the Etheriae again have vesicular cavities interposed between 

 the testaceous laminae ; but the most remarkable example of 

 the camerated structure of the shell is presented by the Spon- 

 dylus varius, Sow. 



It was first noticed, I believe, in the present species, by Mr. 

 Sowerby, in the ' Appendix to Stutchbury's Sale Catalogue,' 

 but the cameration of the lower valve especially approaches 

 so closely to that of the true polythalamous shells, that I am 

 induced to hope that the accompanying description and fi- 

 gures, with the analysis of the fluid contained in the deserted 



