SHELL. 



of the inner layer. These prisms are trans- 

 parent, and polarize light, possessing a crys- 

 talline structure, although their forms are 

 not crystalline but those resulting from 

 mutual pressure. Transverse sections of 

 the prismatic structure exhibit a cellular 

 appearance (PI. 45. figs. 4 & 11 a) ; and a 

 somewhat similar appearance is presented 

 by perpendicular sections (figs. 5 and 11 &). 

 The prisms are pretty easily separable in 

 the lines of mutual contact, and often form 

 several superimposed strata. They fre- 

 quently contain pigment, either uniformly 

 diffused through their substance, or in gra- 

 nules. They also sometimes appear trans- 

 versely striated. 



The inner laminated portion, which some- 

 times constitutes the entire shell, is either 

 white or presents the brilliant iridescent 

 tints of nacre or mother-of-pearl. It is 

 often called the nacreous portion, or nacre, 

 and when polished forms the mother-of- 

 pearl of the shops. Under the microscope 

 it exhibits a number of fine lines or grooves, 

 running in various directions, and probably 

 corresponding to the edges or intersections 

 of the strata or laminae of which this por- 

 tion of the shell is composed ; and it is to 

 the interference of light ensuing at the sur- 

 faces of these grooves that the iridescent 

 colours are usually owing. 



In some shells (Terebratulce) there are 

 tubes traversing the substance perpendicu- 

 larly (PI. 45. fig. 7) or obliquely, or forming 

 branched horizontal channels (fig. 9 a, b) ; 

 in the latter case they are sometimes con- 

 nected with rounded cavities (fig. 9 a). 



In some Gasteropoda, as Cyprceo, the 

 outer portion of the shell consists of three 

 layers of similar prismatic structure, but 

 with the prisms in each layer in alternately 

 contrary directions. The same may be seen 

 in some of the outer layers of oyster-shell, 

 except that the prisms are nearly horizontal 

 or slightly oblique. But in the Acephala 

 generally, the structure corresponds to the 

 inner portion of that of the Cephalophora. 



Shell consists of an organic basis, in which 

 calcareous matter, principally composed of 

 carbonate of lime, is deposited; and by 

 digesting it with dilute muriatic acid, the 

 latter may be removed, an organic cast of 

 the original being left. On treating a thin 

 plate of nacre in this way, Carpenter found 

 that the iridescent colours remained visible 

 until the membrane was stretched and the 

 supposed folds obliterated, when they va- 

 nished ; hence this author concludes that 



[ 692 ] SHELL. 



the edges of the folds were the cause of the 

 interference of light producing the colours. 

 It appears to us, however, objectionable to 

 this view, that the same structure and 

 colour are produced by laminated calcareous 

 and organic matter artificially formed ; that 

 they are also present after tne edges of the 

 folds must have been ground awav, as in 

 sections; and that the colours, in 'the in- 

 stance mentioned, might have been those of 

 a thin plate, and some of the colours of 

 iridescent shell are known to be those of 

 thin plates. It may be stated here that 

 Carpenter considers the lines or striae in 

 nacre to be produced by the edges of folds 

 of a single layer of membrane, arranged so 

 as to He over each other in an imbricated 

 manner. The same author views the shell 

 of the Mollusca as corresponding to the 

 epidermis of the higher animals, calcified. 



The outer prismatic layers of shell are 

 secreted by the borders or margins of the 

 mantle, whilst the inner laminated portions 

 arise from the outer surface. The growth 

 of shell is not uninterrupted or constant, but 

 periodical; hence the laminated arrange- 

 ment of its constituents. 



In some portions of the shell of the 

 oyster, &c., the calcareous matter assumes 

 the form of distinct rhomboidal or hexago- 

 nal crystals (PI. 45. fig. 10). These appear 

 to be deposited in the inner laminated por- 

 tion ; and when detached they leave angular 

 spaces corresponding to them in form. In 

 the tooth of the shell of Mya, groups of 

 radiating prisms are present, forming an 

 elegant microscopic object. 



The prisms existing in the outer portion 

 of shells have been supposed to represent 

 cells filled with calcareous matter; they 

 have also been regarded as consisting of 

 aggregations of epidermic cells, the trans- 

 verse striae (in Pinna) corresponding to 

 thickenings of the cell-membranes where 

 the layers come into contact ; and the folded 

 membrane has been compared to a basement 

 membrane. It is probable, however, that 

 shell should be regarded as a simple secre- 

 tion from the mantle, and as corresponding 

 in structure to egg-shell. 



Shell of the Crustacea. The hard portion 

 of the integument of the Crustacea, alluded 

 to at p. 215, possesses a laminated struc- 

 ture, corresponding to periods of growth, 

 and giving rise to the appearance of trans- 

 verse parallel lines in a perpendicular section 

 (PI. 45. fig. 16). The substance is traversed 

 by numerous straight, or slightly wavy, 



