OCR Text |
Show 1892.] STRUCTURE OF THE SHELL IN VELATES CONOIDEUS. 535 appears in the main to consist of calcic carbonate, since it dissolves with effervescence in dilute hydrochloric acid, leaving, however, an appreciable residue. This residue subjected to the usual tests, both with acids and under the blowpipe, proved to consist of silica, having a specific gravity which is nearer that of the crystalline than the amorphous state. Naturally it was at first thought that this silica might be a product of fossilization, but since the callus, which was equally exposed to the same influence, yields no appreciable residue, this does not appear to be a tenable supposition; at the same time without further and more extended inquiries one hardly likes to look upon it as an original product of the animal1. The crystalline layer which forms the principal thickness of the shell-wall is composed of a single stratum of laminae, the component fibres of which in each successive lamina run in a reverse direction to those of the preceding one, as originally described by Count Bournon2 and subsequently by all writers on molluscan shell-structure. The direction these plates take, however, in the present example is peculiar. In that part where the growth is normal (see fig. 21) their direction coincides with that of the lines of growth, their planes being perpendicular to the outer surface of the shell, just as seemingly obtains in an ordinary Neritina (e. g. JV. gagates). In the remaining portion of the shell-wall beneath the periostracal layer the plates follow the curve of the shell, their planes radiating from the new apex and consequently being approximately parallel with the outer surface-approximately, because the sections show that in each major group of layers they ' feather' somewhat (figs. 25, 26, 27). These walls being, as already mentioned, hewn out of successive margins of former callus, it follows naturally that in the outer margin of the callus itself the plates follow the same course-that is to say, are disposed in a crescent, at the extremities of which, their planes of inclination twisting to suit, they unite with those of the outer lip to form a continuous circle. Along the dentate columellar lip they also run parallel with the margin, and here, as elsewhere over the callus, their planes are at right angles to the exterior surface. O n reaching the posterior angle of the aperture this series of lamellae (viewed from the exterior) abruptly bifurcates, one set curving sharply towards the outer lip, the other in the opposite direction, and both commingling with, and becoming lost in, the marginal plates ; the point of junction of the two series thus forms a centre whence they stream off in three directions (fig. 28). At the anterior angle the whole series curves towards the outer lip, becoming lost, as before, in the marginal set. Across the central portion of the callus they run in an oblique direction, radiating from 1 Schinidel noted that this layer did not appear to be entirely composed of lime. 2 ' Traite complet de la Ohaux carbonate^,' etc. torn. i. (1808) p. 310. See also Gray (J. B.), Phil. Trans, cxxiv. (1833) p. 789 ; Bowerbank (J. S.), Trans. Micro. Soc. i. (1844) p. 128, pi. xv. fig. 1; and Rose (Gl-.), Abhandl. k. Akad. Wissensch. Berlin, 1858 (1859), p. 89. pi. iii. |