1878.] 



SHELLS OF CEPHALOPODS. 



967 



as, e. g., Fistulana clava, Vermetus giyas, Helix decollata, theOstre<e, 

 Estherice, and other Bivalves, more especially the Spondylus varins 1 . 



In Fistulana and the gastropodal Vermetus, the animal periodically 

 withdraws itself from its dwelling-chamber ; the growth of the walls 

 i3 continuous and uninterrupted ; but a thin new floor is formed at 

 some distance from the old one left behind, and a series of chambers 

 fig. 1, a a, results. If the calcareous deposit had been continuous in 

 every part of the shell, a solid tract would have been left behind, as in 

 Magilus. The successive floors or "septa" in Vermetus (fig. 1,6 b), 

 extend freely across and are concave toward the outlet ; they are 

 entire and adherent only by their marginal circumference to the 

 shell- wall (A, B). The contents of the chambers in the living 

 Vermetus are unknown. Both chambers and partitions are the con- 

 sequence of a mode of shell-growth ; physiological ken stretches not 

 beyond this. 



In Spondylus varius (fig. 2) the "septa" are not continued freely 

 across the shell, but are united together near the middle or centre 

 of their extent, at the position of the impression of the adductor 



Fi«-. 2. 



Spondylus varius. 

 Section of chambered part of shell. 



muscle. This, in the forward movement of the mollusk, does not 

 quit its attachment to the nacreous layer of the valves ; whilst the 

 pallial lobe, except at its circumference and where it adheres to the 

 adductor, can and does detach itself from the surface of the valve 

 about to be abandoned, in the progressive growth of the visceral 

 mass. The mantle at each period of repose, then secretes on the 

 fluid occupying the deserted part of the shell, the new septum or 



1 Proc. Zool. Soc. 1837, p. 63. See also H. Woodward, " On the Struc- 

 ture of the Shell of the Pearly Nautilus," Report of the British Association, 

 Liverpool, 1870, Trans, of Sections, p. 128. 



63* 



