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Show 1892.] STRUCTURE OF THE SHELL IN VELATES CONOIDEUS. 537 both, however, there is no change in the direction of growth, and though the peristome is complete in the mature individual it does not increase equally all round as in Velates conoideus, but develops most rapidly at its peripheral point, the diametrically opposite edge of the callus scarcely being added to at all and thus constituting the axis on which the direction of growth pivots. A section taken in the plane of the direction of growth displays the successive layers of former callus spread out like a fan at this point (fig. 30). Judging from external appearances there are but three forms which at first sight would seem likely to exhibit the same structure and mode of growth as Velates conoideus, viz.: Tomostoma neritoides, Desh., from the Eocene, the Oolitic genus Pileolus, and Velatella carditoides from the American Cretaceous : all three are, so to speak, like half-decked limpets in form and increase radially by the addition of fresh material to the continuous peristome; but in neither the first nor last-named, certainly, does the callus in any way overlap the lateral walls of the test, nor is there any but the slightest change in the direction of growth and that at a very early stage. A rough section of the first, however (fig. 31), shows that whilst the callus moves forward in the manner indicated above, the adjacent portion of the wall of the test though continuous with it is probably normal in its growth. Unfortunately the species is a small one and material is scarce, whilst the shell is so fragile that the micro-section which was being prepared crumbled in the final process, and Mr. Riley, to whose skilful manipulation is due the successful series of interesting sections on which this paper is in part founded, was too disheartened by the failure to essay any further attempts. With regard to Pileolus it is much to be regretted that no definite opinion can be formed, since the specimens are so completely mineralized that all intimate structure is lost. Sowerby speaks of breaking-open one and discovering its internal spirel ; but though Dr. Woodward most kindly had two specimens cut, no such trace of any spire is visible in them, the only peculiarity being a thickening of the columellar lip on its inner margin. It was impossible even to make out if there is a septum or not, and it is necessary to await the discovery of better preserved specimens before attempting to pronounce further upon this genus2. As for Velatella, neither the original description 3 nor Tryon's figure 4 enables one to form any conjecture as to its probable mode of growth. 1 ' Genera of Eecent and Fossil Shells.' 2 Since the foregoing paragraphs were written, Mr. W . H . Hudleston, F.E.S., the President of the Geological Society, has most obligingly lent m e a section of Pileolus which he possesses and which comes from the Lower Oolite. It shows distinctly that in this genus there most clearly is a septum, as in Neritina crepidularia and Tomostoma. The Museum sections must have passed just through the spreading base of this septum at its inner margin where it joins the callus. There is no true internal spire as stated by Sowerby. The shell in this specimen was, as usual, far too altered to exhibit any trace of structure. 3 Ann. Rept. U. S. Geol. and Geog. Surv. 1872 (1873), p. 499. 4 Struct, and Syst. Conch, pi. lxxviii. fig. 85. |