Shell-growth in Cephalopoda. 37& 



rather that the imbricating laminae of the pad of the Sepia 

 may be homologous with the imbricating lamineeof the nacreous 

 or middle layer of the Nautilus, but far more loosely aggre- 

 gated, and thus that the Sepia, instead of being a closed tube, 

 like the Nautilus or Belemnite, is an open or boat-like struc- 

 ture ; and this would account for the absence of any cap or 

 nucleus in the former — those structures being essentially rela- 

 ted to the formation of a closed tube — and their homologues 

 would be sought in the open spoon-like commencement of the 

 Sepia, which is clothed round by the mucro. In this view 

 the outer layer of the Nautilus-shell would be represented by 

 the middle layer of the Sepia, a correlation less difficult on 

 account of its excessive thinness in Ammonites and Belem- 

 nites ; and hence the guard of a Belemnite and the shagreen- 

 layer of Sepia are adventitious superadded structures, unrepre- 

 sented in Spirula and probably in Loligo. 



If there is any truth in this, then the approximation or 

 otherwise of the septa of the Nautilus will have little to do 

 with the question ; moreover, if it has, the observations of 

 Mr. Bather are too partial to be of value^ either in relation to 

 the living Nautilus or its extinct congeners. One would sup- 

 pose from his statement that in a Nautilus the earlier septa are 

 approximate, the middle ones far apart, and the later ones 

 approximate again, and he derives a confirmation from this of 

 the theory, which is doubtless often true^ that the characters 

 of senility resemble those of youth. As to this I may quote 

 the following passage from p. 30 of my work : — " Starting 

 from the first septum, we do not find them at distances con- 

 stantly proportional to the diameter. If they were, there 

 would always be the same number in each whorl. In an 

 example, however, of Nautilus pompilius the first whorl has 

 eight chambers, the next sixteen, and the last half whorl 

 seven. The same want of regularity is found in other Nauti- 

 loids. As a rule the earlier septa are more remote, and the 

 middle ones only retain for some distance their proportionality. 

 The last two or three septa of the adult very commonly differ 

 in distance from the rest. In some rare cases they are more 

 remote, but they are usually closer, and the commonest case 

 is that in which the last one is at half the usual distance." 



Again, as to the history of any genus. I should like Mr. 

 Bather to look at the septa of Goniatites Sagittarius of the 

 Devonian, so crowded one can scarcely count them — yet it is 

 not the earliest Goniatites by far, nor yet the last — and then 

 see if he can maintain the statement that " so early as the 

 Goniatites the septa are far apart in proportion to the diameter 

 of the whorl." In fact the distance of the septa seems to be 



