Shell -growth in Cephalopoda. Zll 



Whether the successive layers of the spongioid-tissue or 

 " pad " of the Sepion is formed by intussusception, as 

 Riefstahl says, or by successive formation of chitinous mem- 

 branes by the epithelium of the shell-sac, as Bather says, is, 

 after all, only a matter of argument, in which the latter seems 

 to me to have the best of it. If we are to get any further, 

 we must have some evidence of the homology between these 

 layers and the parts of any other Cephalopod. It is assumed 

 til at these layers correspond to the septa of a Nautilus or of 

 the phragmocone of a Belemnite. But do they ? They have 

 no siphuncle, and they are not even perforated. Yet a si- 

 phuncleand neck are present inNautilus, Ammonite, Belemnite, 

 and Spirula, and, what is more remarkable, though the 

 Nautilus commences with a cap and the other three have a 

 well-marked nucleus, nothing of this sort has yet been dis- 

 covered in Sepia. I specially looked for it, but could find no 

 representative of such a structure. On the other hand, if 

 Mr. Bather had availed himself of my observations of the 

 shell of Nautilus, of which he seems to be ignorant, or had 

 made observations for himself, he could not have written as 

 he does, nor could M. Hiefstahl have supposed for a moment 

 that the Nautilus-shell grows by intussusception between the 

 septa. On page 17 et seq. of my work I give a very 

 detailed description of the structure of this shell. 



I there show that the shell proper is composed of three 

 layers : the ' outer, a porcellaneous one, is formed of large 

 radiating crystalline particles set in a dark ground-mass, 

 which, if they show any orientation, are perpendicular to the 

 surface ; and this layer is not at all divisible into laminaa. 

 It shows lines of growth on the surface, and these pass 

 marginally, and not superficially, into any fractures that may 

 have taken place in the growing edge of the shell. These 

 characters leave little doubt that this layer is formed by 

 secretion at its bounding edge. Very different is the middle 

 layer. It has, as Hyatt pointed out, an imbricated structure, 

 a structure which might very well suggest the pad of the Sepion 

 as its homologue. This imbrication is in truth excessively 

 fine, as about a thousand fine laminee may be counted in its 

 thickness j the outcropping edges of these, being about 

 20,000 to the inch, diffract the light and give rise to the 

 nacreous lustre, when the innermost layer is absent or worn 

 away. The direction of these laminae is outwards in the 

 direction of the aperture of the shell. Their obliquity is very 

 slight, so that in tracing them from their commencement inside 

 to their termination against the outer layer of the shell, they 

 pass more than one septum, and must therefore have been 



