SHELL. 



565 



posed of thin laminae placed side by side, 

 which separate from one another in the planes 

 of cleavage when the shell is fractured. As 

 first pointed out, however, by Mr. Bowerbank, 

 each of these laminae really consists of a series 

 of cells in close apposition ; and the plates 

 are disposed alternately in contrary directions, 

 so that each series of cells intersects the one 

 beneath it nearly at right angles, as seen in 

 fig. 420. 



Although the intimate structure of 



Fiq. 4-20. 



art ion of fractured surface of middle layer of Cyprcta 

 mauritiana, showing laminae, composed of prismatic 

 cells obliquely crossing one another. Magnified 235 

 diameters. (After Bowerbank.) 



each of the three layers of the shell is essen- 

 tially the same, yet the disposition of the 

 laminae is not the same in any two adjoining 

 ones, an arrangement which adds greatly to 

 the strength of the shell. The planes of the 

 laminae are always as nearly as possible either 

 parallel or at right angles to the lines of 

 growth ; those of the inner and outer layers 

 always having the same direction with each 

 other, but those of the middle layer being set 

 at right angles to them. When, therefore, a 

 section is made parallel to the surface of the 

 shell, it will cut the edges of the laminae of 

 which the layers traversed by it are composed; 

 but if the section be made in a direction per- 

 pendicular to the surface, and pass through 

 the middle layer in the plane of its laminae, it 

 will cut through the edges of the laminae 

 making up the interior and exterior layers ; 

 whilst if the section traverse the two latter in 

 the plane of their laminae, it will cut across 

 the laminas of the middle layer. 



The principal departures from this plan of 

 structure are seen in Patella, Chiton, Ha- 

 liotis, and Turbo and its allies. In Patella, 

 the inner and outer layers are composed of 

 large and irregular laminae, by no means 

 firmly adherent to one another ; but the 

 middle layer is made up of tolerably regular 

 polygonal cells, which form only a thin 

 stratum in some parts, whilst in others they 

 are elongated into prismatic cells ; and the 

 directions of the laminae, of which the inner 

 and outer layers are composed, instead of 



being conformable with each other, are at 

 right angles. In Chiton, the external layer, 

 which seems to be of a delicate fibrous tex- 

 ture, but which is of extreme density, is per- 

 forated by large canals, which pass down 

 obliquely into its substance, without pene- 

 trating, however, as far the middle layer. 

 The middle layer, as in Patella, is distinctly 

 cellular ; whilst the internal has the same 

 nearly-homogeneous texture as the external, 

 but shows no trace of perforations. The 

 peculiarities of structure presented by Ha- 

 liotis have been already described. In Turbo 

 and its allies, the inner layer is nacreous, and 

 the middle one is made up of large cells : the 

 cellular structure is also very evident in the 

 solid operculum of Turbo, when reduced to 

 sufficient thinness. 



That the shell-substance in Gasteropoda is 

 formed in the first instance by the agency of 

 cells, however indistinct their traces may 

 subsequently become, is further apparent 

 from the researches of Mr. Bowerbank on 

 the growth of the shell of the common 

 garden-snail (Helix aspersa) ; and his obser- 

 vations further confirm the opinion already 

 expressed, that the formation of each layer 

 of shell is a progressive operation ; new 

 matter being added to its interior after the 

 exterior has been consolidated. 



Passing by the Pteropochi, whose delicate 

 membranous shells present no very distinct 

 structure, we come to the testaceous Cephalo- 

 poda, of which there are but few species now 

 existing. The shell of Nautilus pompilius 

 bears more resemblance to that of bivalves 

 in its intimate structure, than to that of the 

 Gasteropodous univalves ; the three layers of 

 perpendicular laminae, so characteristic of the 

 latter, not making their appearance here; 

 and of the two layers of which the shell is com- 

 posed, the inner one being nacreous, whilst 

 the outer one is made up of an aggregation 

 of cells of various sizes, those which are 

 nearest the external surface being generally 

 the largest. In the thin shell of Argonauta, 

 the same kind of irregular cellular structure 

 can be easily distinguished, as in the outer 

 layer of the shell of Nautilus ; but there 

 would seem to be nothing comparable to the 

 inner layer of the latter. The shell of Spi- 

 rula must be considered to bear a greater re- 

 semblance, as regards its relation to the ani- 

 mal, to the Sepiostaire of the Cuttle-fish, 

 than to the chambered shell of the Nautilus ; 

 although it so closely approximates the latter 

 in its own conformation. This being the 

 case, it is interesting to find that the intimate 

 structure of the shell has a much greater 

 resemblance to the Sepiostaire than would be 

 supposed from its general aspect. For al- 

 though its texture seems uniform, and its 

 minute parts are composed of an aggregation 

 of calcified cells, yet its surface is marked by 

 sinuous lines, closely resembling those which 

 are seen upon the transverse plates of the 

 Sepiostaire ; and these lines or bands project 

 in such a degree, that they might be con- 

 sidered as rudiments of the vertical partitions 



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