OCR Text |
Show 530 MR. B. B. WOODWARD ON THE GROWTH AND [June 14, Where the free edge is thickened and pillar-like, as in JV. intermedia, JV. punctulata, JV. fiuviatilis, and, to a lesser degree, in JV. latissima, the angle it makes with the septiform columellar lip approaches the perpendicular ; where, however, the septum is widely separated from the paries, as in JV canalis and the species that follow in the series quoted above, its free edge is thin, sharp, and inclined more out of the perpendicular, the whole septum sloping back from the aperture. Towards the middle in JV. canalis the free edge of the septum has a somewhat acuminate projection corresponding to that noted above on the columella-edge in JV. cornea, &c. Viewed through the aperture of the shell only a very small portion of the septum can in these latter cases be seen; in its earlier stages in the other species cited it is of course quite out of sight. The shifting of the septum away from the parietal wall is due to a purely mechanical cause. Just in proportion as the whorls of the shell increase more rapidly and the spire at the same time becomes more depressed, so the latter approximates the posterior angle of the aperture, and consequently the inner wall of the last whorl becomes smaller and smaller and less and less adapted to serve as a myophore. Moreover, as the shell becomes flattened and the septiform columellar lip extends further and further outwards from the axis, the angle at which the retractor muscle would have to act in order to withdraw the extended animal, were its fixed point still the remnant of the paries, would be one of great disadvantage from a mechanical point of view, and hence a fresh attachment, as near to and as much above the aperture as possible, becomes of great importance and is obtained by the forward movement of the septum already described. The manner of this forward growth will be best dealt with later on when the growth of the shell of Velates conoideus is under consideration. In all cases, too, the scars of the anterior and posterior retractor muscles are pronounced in proportion to the strength of the muscles that were attached thereto, and, in consequence, to the thickness of the shell and the septum. It must also be borne in mind that the flattened columellar lip characteristic of Neritina and Nerita is formed by the callus, as likewise is the "shelf" in Septaria [= Navicella]. In Nerita the myophore at first sight appears as if it were a remnant of the paries ; but a closer inspection, if one may judge from such typical forms as JV. peloronta, Linn. (fig. 14), JV. polita, Linn., and JV. plexa, Chemn., reveals the fact that it is merely the septum, all trace of the inner walls and columella having been removed. Velates, so far as at present known, is represented by but two species-V. conoideus, Lamk., and V. equinus, Bez., which occur together in the Lower and Middle Eocene of the Paris Basin. The shell of the latter attains to close upon 4 whorls when full-grown, and at that age corresponds in size with specimens of V. conoideus that have only completed about 3 whorls. The internal construction of the two at these respective periods is so similar that the description |