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Show 540 ON THE SHELL IN VELATES CONOIDEUS. [June 14, of w. The cavity extends behind s to the point marked +, and therefore far beyond the sutural line (suture); the outer wall at this point is very thin. Figs. 15-29. Velates conoideus, Lamk. a, apex; c, callus ; s, myophore, which at first is formed by a portion of the paries, but subsequently consists solely of the septum. Fig. 15. Young shell of about 2 whorls, to show the small extent of the callus. X 3. 16. Young shell of about 3£ whorls 18. „ „ 4i 19. „ „ U ',. Apical view, showing proportion of callus. • b. Dorsal view. c. Ventral view. d. Section to show interior. 20. „ „ over 4£ in which fresh periostracal layer begins to be deposited over the callus. ) (Fig. 16d x 2; Fig. 17d x 2; Fig. 18d X l) 21. Adult shell from the anterior side (f nat. size), to show the relative proportion of shell in which the growth is normal to that formed of callus and excavated callus. PLATE XXXII.1 Fig. 22 a. Axial section of young shell of about 3 whorls: X8. p.l., periostracal layer; m.l., middle or crystalline layer; i.l., infilling layer; c, callus. The portions of pre-existing layers belonging to the young shell and included in the present walls are marked by the same letters with the addition of accents. 22 b. Portion of the anterior wall further magnified, to show the structure p.l. and the lines of growth in m.l. 22 c. Junction of the outer wall and callus on the anterior side of the same section. 23 a. Axial section of a specimen of about 3^ whorls : X 6. 23 b. Junction of the callus and outer wall on the anterior side, further magnified. 24 a. Axial section of a specimen of 4g whorls : X 3J. 24 b. Following section of the same, showing actual apex. 24 c. Junction of callus and outer wall on the anterior side, further magnified. 25. Section through adult shell in the plane of the septum, to show the connection of the latter with the outer wail. 26. Section parallel with the preceding a little further in from the aperture and viewed in the opposite direction. 27. Radial section of an adult specimen taken on the side away from the aperture and showing the junction of the callus and the outer wall, which consists of layers of former callus. 28. External ventral surface of adult specimen with the course of the component plates of the callus marked out. [From two specimens.] 29. Internal ventral surface from a specimen in the British Muse um (Natural History) [Reg. 31977], showing the courseof the component plates and their relation to the septum. 30. Section of a portion of Neritina crepidularia, Lamk., in the plane of the direction of growth, showing the junction of the callus and outer wall of the shell on the side furthest from the aperture (considerably enlarged). 31. Similar section of Tomostoma neritoides, Desh. (considerably enlarged). 1 The inicrostructure of the sections is filled in diagrammatioally. |