534 MR. B. B. WOODWARD ON THE GROWTH AND [JuHC 14, 



was callus (c'), as shown by the structure, and has been shaped to 

 suit its present position, as the transverse direction of its lines of 

 growth testify. The wall on the posterior side of the section is 

 built up of an old whorl, of callus, and of infilling material. The 

 absolute apex of each of these two last specimens appears in the 

 succeeding section of the respective shells, and here may be seen the 

 remains of the true spire, such as it is, and the overlapping sutures 

 (fig. 24 b). 



These remnants exist in the adult shell, unless indeed it be a 

 worn specimen ; all other traces of former walls have disappeared. 

 A section taken through a full-grown specimen as nearly as possible 

 in the plane of the septum (fig. 25), and stained with picro-carmine, 

 clearly reveals the intimate relationship between the myophore and 

 the neighbouring walls of the test and demonstrates beyond doubt 

 the way in which they have been carved out of successive layers of 

 callus, for the lines of growth can be traced right across the septum 

 to its inner margin and reappear in the wall of the shell on the 

 opposite side of the cavity (see also fig. 26). They may also be 

 distinctly seen running round the walls on the interior surface of the 

 sheir. 



The composition and intimate microscopical structure of the 

 Velates shell likewise call for some remark, since further points of 

 interest are presented by them. 



The term periostracal layer has been employed throughout, 

 instead of periostracum, for the external layer, the reason being that 

 it differs so widely in its composition from the chitinous substance 

 which coats most shells, and which usually is known by the latter 

 term, that its application in this instance might have been misleading. 



This layer is remarkably hard and extremely difficult to cut 

 through ; it does not stain like ihe rest of the shell, but remains 

 translucent with a cloudy fibrous appearance when seen in the 

 microsections already described. It retains probably to a very high 

 degree (especially in the young shell) the coloration and mottled 

 markings which adorned the shell when living. Its line of junction 

 with the underlying crystalline layer in the young shell and that 

 portion of the adult where the growth is normal is even ; but 

 where it overlies callus the several layers of the two inosculate and 

 the line of junction is extremely ragged and irregular (figs. 24 and 

 27). In the one case the mantle-edge that secreted it was advancing 

 over a given area followed in regular order by the portion that 

 furnished the material for the second layer ; in the other the mantle 

 was retreating from, the area of deposition as the shell increased in 

 size and so gave rise to overlapping of the two series of deposits, 

 the irregularities that thus arose being further complicated by the 

 fluctuations in the growth of the animal and its shell. 



Under the delusion that this periostracal layer might, like the 



niolluscan radula, prove to be very dense chitine, a portion was 



placed in Mr. G. T. Prior's hands for examination. He most 



kindly made a careful investigation of it, with the result that it 



^ These lines of growth were noted by Schmidel. 



