162 M. Necker on the MinercUogical Nature of Shells. 



its gi-eat fragility). MytU/us edulis, (strongly). Lutraria elegans, Fleming, 

 (strongly). Mya truncata, (strongly). Mactra stulturim, (strongly, although 

 fragile). Cardium acidmtum, (strongly). Cypriha islandica, (strongly). Ve- 

 nerupis perforans, (strongly). Pectcn opercularls, (more or less strongly). 

 Sohn siliqua, (not strongly, although thick). Solen ensls, (strongly, although 

 fragile). Baktnns (?) (strongly). Fhokis crispata, (strongly). 



It is remarkable, that two genera of perforating shells, the 

 Pholas and the Venenipis, scratch calcareous spar strongly. 

 Thus, the rugosities with which their shells are provided may 

 assist, along with the acids with which they are fm-nished, to 

 excavate the calcareous rocks which they inhabit. The idea 

 that these shells were composed of carbonate of lime, seemed 

 to render it impossible that they should perforate calcareous 

 rocks, whose hardness was supposed to be equal to their own. 

 It is now evident, that, as they are composed of arragonite, 

 they may act mechanically even on the hardest limestones. 



If we add to this hardness a specific gravity also higher 

 than that of calcareous spar, as found by M. de la Beehe, 

 there can be no doubt that the substance of most of these 

 shells is arragonite. In fact, this specific gravity is, in the 

 shells determined by M. de la Beche, in most instances higher 

 than 2.7, which is that of calcareous spar, and is even as high 

 as 2.8 in one case. The specific gravity of arragonite is 2.9 ; 

 but we must recollect, that in shells the calcareous mineral 

 matter is always mixed with organic matter, whose density 

 must be very inconsiderable, and thus proportionably dimi- 

 nish the specific gravity. 



Perhaps this last-mentioned matter exists in a greater pro- 

 portion in those shells cited by M. de la Beche, whose specific 

 gravity is below 2.7. Perhaps, also, calcareous spar may, to 

 a certain extent, enter into the composition of certain species 

 of shells ; and this would explain how a large Strombus, af- 

 forded to the Count de Bournon, in an accidental fracture, the 

 incidences of the faces of the primitive rhombohedi'on of cal- 

 careous spar. The two layers, of which certain shells are 

 composed, as the Anadontas and the TJnios, may be, the one 

 calcareous spar, and the other arragonite. If, then, the 

 Strombus, cited by the Count de Bournon in his work on car- 

 bonate of lime and ai'ragonite, should be of the same nature 



