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Show 1892.] STRUCTURE OF THK SHELL IN VELATES CONOIDEUS. 533 the shell, but to fill up the space no longer occupied by the animal and to thicken the shell where, having been formed when the animal was young, it was too thin for the present requirement of its occupant. The periostracal layer, which shows a tendency to divide into two zones (fig. 22 6), is translucent and presents to the eye a fibrous structure, the fibres being arranged at right angles to the surface in the region of the apex; but as the layer is followed down towards the anterior end of the columellar lip they change their direction gradually till they assume the feather-like appearance familiar in shell-structure and are arranged in a direction approximately parallel with the layer itself (fig. 22 c). The second principal layer also exhibits a fibrous appearance consequent on the arrangement of its crystalline plates, which near the apex are nearly but not quite a: right angles to the surface. As the columellar lip is approached these become more inclined, till on joining those of the columellar lip itself they unite with them and sweep through an arc of more than a quarter of a circle (fig. 24 c). The layers of growth can in places be clearly seen (figs. 22 a, b). The innermost deposit is confined to the neighbourhood of the apex and thins out as it recedes therefrom : it shows in section the cross-hatched structure so common in sections of shell and due to the especial arrangement of its component plates. That the myophore at this stage is still formed in its upper part of the parietal wall is evident from its structure and its continuity with the outer wall next the apex (fig. 22 a). The middle portion of the myophore and the posterior wall have unfortunately been broken away in grinding this section. Nevertheless sufficient of the base of the former and of the callus out of which that base has been formed is left to show that even at this early age a considerable enlargement of the interior by absorption has taken place. In the next older specimen, one of about three and a half whorls (fig. 23 a), the remnant of the parietal wall can be traced, the section beingperfect, for quite two-thirds of the length of the myophore, whilst in the outer wall on the posterior side of the section the remnant of another original wall is present. A portion of the periostracal layer, about halfway down that side, curves inwards, traverses the shell-wall, and abruptly terminates on the inner side (p'.l'.): it is overlain by the edge of the callus which comes up to this point and which is in its turn covered by a fresh deposit of periostracal layer that joins on to the first one. This junction is not a true suture, since it does not mark the union of two whorls. The extent to which the callus has been absorbed on its inner side is clearly shown, as also the vertical arrangement of its component plates. This structure, however, is still better seen in more advanced stages of growth. The third section (fig. 24 a) has been taken of the young shell at the stage when its four and a half whorls Jiave been completed and the period of radial growth entered on, just at the time when the callus having attained its maximum development begins to be covered all round its margin by the periostracal layer. The myophore is still in part formed of an old outer wall of the test, but that portion |