SHELL. 



559 



striation is seen to be a thickening of the cell- 

 wall in those situations ; which will of course 



Fig. 411. 



Calcareous prisms of the shell of Pinna from Chalk. 



produce a corresponding series of indentations 

 upon the contained prisms. This thickening 

 seems best accounted for by supposing (as 

 first suggested by Prof. Owen) that each long 

 prismatic cell is made up by the coalescence of 

 a pile of flat epidermic cells, the transverse 

 striation marking their lines of junction ; and 

 this view corresponds well with the fact that 

 the shell-membrane not unfrequently shows a 

 tendency to split into thin laminae along the 

 lines of striation, as shown in the lower part 

 of Jig. 410; whilst we occasionally meet with an 

 excessively thin natural lamina, composed of 

 flat pavement-like cells resembling those of 

 the epithelium of serous membrane, lying 

 between the thicker prismatic layers, with one 

 of which it would have probably coalesced 

 but for some accidental cause which pre- 

 served its distinctness. That the entire length 

 of the prism is not formed at once, but that it 

 is progressively lengthened and consolidated 

 at its lower extremity, would appear also from 

 the fact that where the shell presents a deep 

 colour (as in Pinna nigrina) this colour is 

 usually disposed in distinct strata, the outer 

 portion of each layer being the part most 

 deeply tinged, whilst the inner extremities of 

 the prisms are almost colourless. 



The prismatic arrangement of the carbonate 

 of lime in the shells of Pinna and its allies 

 has been long familiar to conchologists ; but 

 it has been usually regarded as the result of 

 crystallisation. It is now, however, perfectly 

 evident that the calcareous prisms are nothing 

 else than casts of the interior of the prismatic 

 cells; the form of which, however irregular, 

 they constantly present ; whilst the markings 

 of the membrane are faithfully transferred to 

 the surface of the prism. Further, the prisms 

 in a thick layer of shell frequently present a 

 decided curvature, which would not be the 

 case if their form were due to crystallisa- 

 tion. Not unfrequently, moreover, thev are 

 altogether destitute of angular boundaries ; 

 the large quantity of animal matter disposed 

 between the contiguous cells giving them a 

 rounded contour, as seen in^g. 412, and thus 

 causing the calcareous casts of their interior 

 to be cylindrical rather than prismatic. 



It is only in a few families of Bivalves, how- 

 ever, that the cellular structure is seen in this 

 very distinct form, or that it makes up a large 

 part of the substance of the shell ; and these 

 families are for the most part nearly allied to 

 Pinna. In all the genera of the Margaritaceee, 

 we find the external layer of the shell formed 

 upon this plan, and of considerable thickness ; 

 the internal layer being nacreous. In the 

 Unitmidcc, on the contrary, nearly the whole 



thickness of the shell is made up of the inter- 

 nal or nacreous layer ; but a uniform stratum 



Fie. 412. 



Lamina of outer layer of shell of Ostrea edulis, 

 showing its cellular structure, with a large amount 

 of intercellular substance. Magnified 2oQ diameters. 



of prismatic cellular substance is always found 

 between the nacre and the periostracum. In 

 the Ostracece the greater part of the shell is 

 composed of a sub-nacreous substance, the 

 successively-formed laminae of which have very 

 little adhesion to each other ; but every one 

 of these laminae is bordered at its free edge by 

 a layer of the prismatic cellular substance, 

 distinguished by its brownish-yellow colour: 

 this structure presents itself again in the family 

 Pandoridce, which belongs to quite a different 

 section of the class ; and it is curious to ob- 

 serve that the marked difference in the struc- 

 ture of the shells of Pandora and Lyonsia from 

 that of the Anatinidcc and other neighbouring 

 families, harmonises completely with the pecu- 

 liar combination of characters presented by 

 the animals of these two genera.* In all the 

 foregoing cases, a distinct cellulo-membranoiis 

 residuum is left after the decalcification of the 

 prismatic substance by dilute acid ; and this is 

 most tenacious and substantial where, as in 

 the Margaritaceee, there is no proper perios- 

 tracum, as if the horny matter which would 

 have otherwise gone to form this investment 

 had been diffused as an intercellular substance 

 between the proper cell-walls. 



In many other instances, a cellular arrange- 

 ment is perfectly evident in sections of the 

 shell ; and yet no corresponding structure 

 can be distinctly seen in the delicate membrane 

 left after decalcification. In all such cases 

 the animal basis bears but a very small propor- 

 tion to the calcareous deposit, and the shell is 

 usually extremely hard. A very characteristic 

 example of this is presented by the outer layers 

 of the shells of the genus Thracia and other 

 AnatlnidcB. But there are numerous other 

 cases, in which no traces of cellular structure 

 can be detected in the fully-formed shell, and 

 in which we can only be guided by analogy in 



* See Forbes and Ilanley'.s British Mollusca 

 vol. i. pp. 207, -213. 



