530 MR. B. B. WOODWARD ON THE GROWTH AND [JunC 14, 



Where the free edge is thickened and pillar-like, as in N. inter- 

 media, N. punctulata, N. jluviatilis, and, to a lesser degree, in N. 

 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 N. canalis and the species that 

 follow in the series quoted above, its free edge is thin, sharp, and 

 incHned more out of the perpendicular, the whole septum sloping 

 back from the aperture. 



Towards the middle in N. canalis the free edge of the septum has 

 a somewhat acuminate projection corresponding to that noted above 

 on the columella-edge in N. 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 

 hy 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 N. peloronta, Linn. (fig. 14), N. polita, 

 Linn., and N. plecca, 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— F. 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 lo close upon 4 whorls when full-grown, 

 and at that age corresponds in size with specimens of F. conoideus 

 that have only completed about 3 whorls. The internal construction 

 of the two at these respective periods is so similar that the description 



