. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 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 cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 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. Fu
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