1892.] STRUCTURE OF THE SHELL IN VELATES CONOIDEUS. 537 



both, however, there is no change ia 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 vrhich 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. : Tomosioma 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 qallus 

 moves forward in the manner indicated above, the adjacent portion 

 of the VFall 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. Biley, 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 spire ^ ; 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 necessaj-y to 

 await the discovery of better preserved specimens before attempting 

 to pronounce further upon this genus ^. 



As for Velatella, neither the original description ^ nor Tryon's 

 figure ^ enables one to form any conjecture as to its probable mode 

 of growth. 



^ ' Genera of Kecent and Fossil Shells.' 



^ Since the foregoing paragraphs were written, Mr. W. H. Hudleston, F.E.S., 

 the President of the Geological Society, has most obligingly lent me 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 

 caUus. There is no true internal spire as stated by Sowerby. The sheU in 

 this specimen was, as usual, far too altered to exhibit any trace of structure 



3 Ann. Eept. U. S. Geol. and Geog. Surv. 1872 (1873), p. 499. 



^ Struct, and Syst. Conch, pi. Ixxviii. fig. 86. 



