OCR Text |
Show 1892.] STRUCTURE OF THE SHELL IN VELATES CONOIDEUS. 531 of one of them is sufficient, and no further reference need be made the growth of the shell in V. equinus since it is perfectly normal. During the early stages of its growth, i. e. up to about 3 or 3| whorls, the myophore in the young Velates shell (fig. 16) is formed of the remnant of the parietal wall strengthened by shelly deposit much in the same way and degree as it is in Neritina virginea. No trace of a prominence is perceptible at first on the thin and sharp columella-edge, but by the time 3| whorls are finished it is plainly discernible, and with the completion of the fourth whorl it attains its maximum development (fig. 17 c?). The angle the columella-edge makes with the columellar lip scarcely if at all exceeds 45° and is less in the earlier stages, whilst the whole myophore rakes back from the aperture and is not visible from without. After the completion of the fourth whorl the growth of the septum is comparatively rapid and increase takes place especially along that portion of its free margin which is furthest from the callus. By the time 4\ whorls are complete the free margin is almost perpendicular to the callus and has become thickened and pillar-like, a slight fold on it marking the position of the late prominence (fig. 18 d). The appearance of the septum at this stage is most like that of Neritina punctulata amongst recent species. Shortly after the completion of 4| whorls (fig. 20 d) the septum alone constitutes the myophore, the columella and paries having been absorbed, and occupies the relative position it retains throughout the remaining period of growth: its free and thickened edge is quite perpendicular to the callus, and the greater portion becomes visible through the aperture, although the whole septum curves inwards as it projects into the chamber of the shell. So far, therefore, as the myophore is concerned the shell of Velates conoideus offers in the growth of the individual a series of conditions which in the recent forms find their parallel in distinct species:-in its earlier stages the paries and the incipieut septum go to form the myophore, as in the instances quoted ; in the later period the septum alone plays that part, as in Neritina crepidularia. The scars of the retractor muscles, both anterior and posterior, become more marked as the individual grows, the former especially deepening with age, and there is a well-marked anterior palatal apophysis which gradually becomes fainter as the shell grows and finally almost entirely disappears. The callus which in the young shell does not extend very far over tbe adjacent portion of the body-whorl (fig. 15) gradually spreads further and further (figs. 16, 17), till by the time 4\ whorls have been completed (fig. 18) it has covered nearly one half of the body-whorl and its line of demarcation is in one plane with that of the margin of the outer lip. The rate of increase of the third and following whorl is, moreover, proportionately greater than that of the preceding ones. It is at this point that the great change in the manner of growth of the shell begins. First of all the callus is greatly thickened till it becomes in proportion to the shell itself larger and thicker than at any subsequent period (fig. 19). In the next place, further increase of the shell begins to be effected by the addition of fresh material |