1892.] STRUCTURE OF THE SHELL, IN VELATES CONOIDEUS. 529 



behind the posterior portion of the columellar Up, 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 XXXI. fig. 1), N. 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 eoluinella-edge of the spiral party-wall {ox 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 N. 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 

 N. 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. N. fluviatilis, Linn. (fig. 7), and an 

 undetermined species (fig. 8) closely allied to N. canalis, exhibit a 

 further stage in which these two partitions stand slightly apart ; and 

 the distance between them is successively increased in N. canalis, 

 Sow. (fig. 9), N. granosa. Sow. (fig. 10), and N. tahitiensis, Lesson. 

 The parietal wall in adult specimens of the last-named disappears 

 entirely : it also is completely removed in N. 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. 



