1892.] STRUCTURE OF THE SHELL IN VELA.TES CONOIDEUS. 531 



of one of them is sufficient, and no further reference need be made to 

 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 d). 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 rf). The ap- 

 pearance 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 incipient 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 the adjacent portion of the body- 

 whorl (fig. 15) gradually spreads further and further (figs. 16, 17), 

 till by the time A\ 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, proportion- 

 ately 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 

 auy subsequent period (fig. 19). In the next place, further increase 

 of the shell begins to be effected by the addition of fresh material 



