426 Oil Shell-growth in Cephalopoda. 



The latter, it is true, said : " In the acute angles made by the 

 junction of the septa with the circumference of the shell is 

 another deposit, less transparent than the nacreous layer, but 

 showing very little structure." It seems as though Prof. 

 Blake were now confusing this with the " third layer." But 

 his Monograph distinctly leads one to understand that this 

 deposit does not pass between the septum and the shell. He 

 mentions also "a loose amorphous deposit'" lining each sep- 

 tum on either side, apparently continuous with that filling the 

 angles. The fact is that all these deposits are of the same 

 essential structure as the nacreous layer and septa. The 

 constituent membranes are less compressed in the angles, but 

 they become compressed and pass between the septum and 

 the previously formed portion of shell-wall. They are, how- 

 ever, united with the septal and shell membranes on either 

 side by transverse chitinous connexions ; these appear to be 

 the walls of what Prof. Blake calls " lacunai " ; they pass 

 right into the nacreous layer and into the septum. There is 

 therefore organic connexion between the septum and shell- 

 wall in Nautilus, just as Biefstahl first described in Sepia. I 

 confess that in my explanation (' Annals,' p. 306) I expressed 

 myself too definitely ; the credit of pointing this out is due 

 to Prof. Blake, but it will be understood that there was 

 nothing in his previous description to conflict with my idea, 

 and that his present statements are too incorrect to influence 

 the same. The following alteration of my previous paper 

 (lb. p. 306) is based on my own observations ; the altered 

 words are in italics : — " On the surface of the cells that 

 coat the visceral hump a layer of chitin * is, by concrescence 

 of their distal portions, continually formed, and from it 

 the membranes are, as it were, exfoliated. Secretion begins 

 in the anterior region of the shell -wall, a7id proceeds back- 

 wards to the suture, thence centripetally over the septum 

 to the posterior margin of the septal neck. The chitin of 

 the septum is essentially one with the chitin of the shell- 

 wall. Prohahly before, but 2^ossibly in consequence of cal- 

 cification^ this chitin splits into membranes [vide supra). 

 Lime is deposited as arragonite upon and between these mem- 

 branes soon after their secretion ; nacre is produced by this 

 more purely physical process, not by direct secretion^ I hope 

 that this theoretical explanation will satisfy Prof. Blake, and 

 I must thank him for affording me an opportunity of making 

 the correction. 



* Chitin, more correctly conchiolin (see footnote, p. 303). 



t See Osborn, Stud. Biol. Lab. Johns Hopkins Univ. ii. p. 427 (1863). 



