1892.] STRUCTURE OF THE SHELL, IN VELATKS CONOIDEUS. 539 



1786. Nerita schmideliana sinistrorsa, fossilis, Chemnitz : 



Mart. u. Chem. Conch.-Cab. ix. Abth. i. p. 130, pi. 114. 



ff. 975-6. 



[" . . . habe ich sie Neritam Schmidelianam genant."] 

 1789. Nerita perversa, Gmelin : Linn. Syst. Nat. ed. xiii. torn. i. 



p. 3686. 

 1804. Nerita conoidea, Lamarck : Ann. Mus. Hist. Nat. Paris, 



V. p. 93. 

 1805. , De Roissy : Soanini's Suites a Buffon ; Hist. 



Nat. Moll. V. p. 273, no. 9. 

 1810. Velates conoideus, Montfort : Conch. Syst. ii. pp. 354-6, 



fig. 

 1822. Neritina perversa, Lamauck : Hist. Nat. Anim. sans Vert. 



vi. pt. 2, p. 183. 

 . Neritina schmideliana, Sowerby : Genera of Shells. 



1824. Neritina conoidea, Deshayes : Descript. Coq. fos-. Paris, ii. 

 pp. 149-151, pi. xviii, [with fig. of operculum]. 



1825. Nerita perversa, De Blainville : Diet. Sci. Nat. xxxiv. 

 p. 477. 



1866. Neritina schmideliana, Deshayes : Anim. sans Vert. 

 Bassin de Paris, iii. pp. 18-19. 



EXPLANATION OF THE PLATES. 



(Unless otherwise stated the figures are of the natural size.) 



Plate XXXI. 



Figs. 1-10. A series of specimens of Neritina, with the outer wall of the bodj-- 

 whorl cut away to show the suecessiTe stages in the formation of the shelly 

 septum (s), which ultimately supplants the inner wall of the whorl, or paries (w), 

 in its function of myophore, and exhibiting their relation to each other and the 

 callus (c). The approximate position of the apex is marked a. 



Fig. 1. Neritina cornea, Linn., in which there is only a small amount of shelly 

 deposit at the junction of w with s, and possibly, very faintly, along 

 the basal portion of the columella-edge of w. 



2. N. gagates, Lamk., in which the shelly deposit is slightly more pro- 



nounced and begins to form a spur (s) up the columella-edge of w. 



3. N. dubia, Ohemn., exhibiting a further thickening of the shelly deposit 



and a strengthening of s. 



4. N. bicolor, Eecl. : s is yet more developed and begins to project beyond w, 



and form a septum, as seen in 4 a, which is a view taken more from 

 the inner side. 



5. N. punctulata, Lamk. : the septum s stands well away from w, and its 



free margin is seen to be greatly thickened when it is viewed from 

 behind, as shown in 5 a. 



6. N. latissima, Brod. : s and w begin to separate. 



7. N.fluviatilis, Linn. : X 2, s and w stand apart. 



I'. KSsfsow. I ' ^?^ Z ^""^ ^''f^^^^''^^^ "^T "^''M^ separated from 

 -in -\T a each other, and c more and more developed. 



10. N granosa. Sow. J ' ^ 



11. iV". crefidularia, Lamk. Section in the plane of the direction of growth : 



w is entirely absorbed and s is placed near the margin of c. 



12. Tomostoma neritoides, Desh., x 3. Section as in fig. 11, with which it 



closely agrees. 



13. Pileolus plicat'us, Sow., X 2. Section as in fig. 11. 



14. Nerita j^eloronta, Linn. Section to show septum s and total removal 



