F. A. Bather — Growth of Cephalopod Shells. 449 



in Sepia the inner surface of the mantle covering the lamellae is formed 

 of columnar cells, which at the anterior margin, where growth is 

 most vigorous, are extremely long ; removing this, there are seen on 

 the ventral side of the last-formed septum, especially at the margin, 

 developing sinuous partitions, over which I have been unable to 

 demonstrate any such conchyolin membranes as are shown in Fig. 2. 

 Further, this hypothesis requires subsequent growth of the septum 

 as well as of the wall. 



In the adult Nautilus, Ammonites, and Spirilla the last septum 

 is always nearer the penultimate septum than the regular increase 

 in the depth of the chambers would lead one to expect ; this also 

 obtains in Belemnites so far as our fragmentary specimens allow one 

 to judge, and, as Eiefstahl has shown, in Sepia. The Intussuscep- 

 tion-hypothesis demands (i) that this should be also the case in the 

 young ; (ii) that, in any one species, the body-chamber should at all 

 ages be of the same depth in proportion to the length of the whorl. 

 Eiefstahl and his reviewer in Naturforscher (April 30, 1887) assume 

 these facts and adduce them as arguments. But in smaller and 

 presumably younger specimens of Sepia the last-formed lamella does 

 not always seem to be nearer the penultimate, while the spaces 

 between the insertion-lines are regular : it is easy to be deceived by 

 the fact that all the lamellae approach one another at their posterior 

 borders. In Nautilus sections through undoubtedly young shells 

 show that the last-formed chamber is actually larger than the pre- 

 ceding. In such young shells a distinct nacreous ring often indi- 

 cates the first formation of a fresh septum. Had it been completed, 

 the chamber enclosed by this septum would, without any interstitial 

 growth, have been of increased size : the ratio of the body-chamber 

 to the whorl, as compared with other specimens, is less or greater 

 according as this half-formed chamber is counted in or not. The 

 evidence from Ammonites is often vitiated by fracture ; granting that 

 the last chamber in small specimens is sometimes of relatively little 

 depth, it is not so always, and in adults there may be seen, though 

 rarely, a small air-chamber between two of normal size. In Nautili 

 the 8th chamber is always less deep than those preceding, for the 

 increased breadth and height of the cone makes up for loss of depth : 

 examination of shells with only 8 or 9 chambers would therefore be 

 deceptive. Eiefstahl says that his fixed point is the hinder end of 

 the animal's body : this does not explain the lines of advance so 

 clearly seen in the muscle-scars. 



So far facts do not confirm the Intussusception-hypothesis, some of 

 those here brought forward favour the old view. Still Dr. Eiefstahl 

 deserves our gratitude for re-opening an interesting question, which 

 is not likely to be settled without further investigation.^ 



^ As bearing upon the subject of this Article, see a paper " On the Structure of 

 Camerated Shells "by Henry Woodward, Popular Science Eeyiew, 1872, vol. xi. 

 pp. 113-120, pi. Ixxxii. 



DECADE III. — VOL. IV. — NO. X. 29 



