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 3'oung, it was too thin for the present requirement of its occupant. 



The periostracal layer, which shows a tendency to divide into two 

 zone's (fig. 22 b), 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 coluinellar 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 e). 



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 columeliar lip is approached these become more inclined, till 

 on joining (hose of the columeliar 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, 5). 

 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 

 being perfect, 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. 



T!ie third section (fig. 24 a) has been taken of the young shell at 

 the stage when its four and a half whorls have 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 



