OCR Text |
Show 1892.] STRUCTURE OF THE SHELL IN VELATES CONOIDEUS. 529 behind the posterior portion of the columellar lip, a septum that takes the place of the columella and serves as the point of attachment for the posterior retractor muscle. This structural alteration is effected not after the animal has completed growth, but proceeds pari passu with it. In Neritina a series of species may be selected which will exhibit stages in the degree of removal of the columella and inner walls of the whorls and in the development of the septum. Neritina cornea, Linn. (Plate X X X I . fig. 1), JV. gagates, Lamk. (fig. 2), and thin specimens of N. zebra, Linn., and N. dubia, Chem., show an early stage. There is no true columella, and some portion of the columella-edge of the spiral party-wall (or paries) separating the whorls has been removed. At the base of the columella-edge where the parietal wall joins the projecting columellar lip the angle is filled in with shelly matter strengthening the union between the two. A slight spur of the shelly deposit runs in some cases (N. gagates and N. zebra) up the columella-edge and supports it. The whole forms a myophore and serves as a point of attachment for the posterior retractor muscle, a slight salient point left in some species about halfway up on the columella-edge of the whorl wall marking the limit of its scar. In thick specimens of N. dubia (fig. 3), in N. smithii, Gray, and in N. bicolor, Reel. (figs. 4 and 4 a), there is a further thickening of the shelly deposit, which begins to spread over the remaining portion of the parietal wall in the direction of the apex. The columella-edge is additionally strengthened in N. virginea, Lamk., and the paries nearly concealed by the increased deposition; whilst in JV. intermedia, Sow., and N.punctulata, Lamk. (figs. 5 and 5 a), this shelly deposit completely covers the parietal wall, stretches out from the columella-edge, and forms a veritable septum reaching one third of the way across the whorl, its free margin becoming thickened, pillar like, and firmly attached to the outer walls of the shell by its spreading ends. The next link in the series is supplied by that very peculiar species JV. latissima, Brod. (fig. 6), in which the septum with its pillar-like margin stands away from the columella-edge, so that although united posteriorly the septum and the remaining portion of the paries project independently into the single chamber of the shell: the septum is now the myophore. JV. puviatilis, Linn. (fig. 7), and an undetermined species (fig. 8) closely allied to JV. canalis, exhibit a further stage in which these two partitions stand slightly apart; and the distance between them is successively increased in JV. canalis, Sow. (fig. 9), JV. granosa, Sow. (fig. 10), and JV. tahitiensis, Lesson. The parietal wall in adult specimens of the last-named disappears entirely : it also is completely removed in JV. crepidularia, Lamk. (fig. 11). The Eocene form Tomostoma neritoides, Desh. (fig. 12), undoubtedly comes very close to the last-named; not only does it resemble it in its external characteristics, but also in the internal arrangement of the septum and the lack of all trace of the parietal wall. One or two other points about the septum are worthy of note. |