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 animal \ 



The crystalline layer which forms the principal thickness of the 

 shell-wall is composed of a single stratum of laminse, 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 

 Bournon" 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 seem- 

 ingly obtains in an ordinary Neritina (e. g. N. 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. 



On 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 



^ Schmidel noted that this layer did not appear to be entirely composed of 

 lime. 



^ ' Traite complet de la Ohaux carbonat6e,' etc. tom. 1. (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 Eose (G.), Abhandl. k. Akad. 

 Wissensch, Berlin, 1858 (1869), p. 89. pi. iii. 



