104 BULLETIN OF THE 



imbricated, and the direction of zone the same as those of the internal 

 layer. 



"When the first revolution is completed, the hood begins to deposit 

 the dark layer on the external surface of the ventral side of the first 

 whorl, and the true external layer becomes very thin, and is often 

 hardly definable, appearing in the section as a very faint band next to 

 the black deposit of the hood. On the ventral side, however, it steadily 

 increases in thickness. It is this layer also which attains such ar 

 excessive development, and composes the principal part of the shell 

 close to the umbilical border. The brownish color of the exterior does 

 not seem to be distinct, but rather to be intimately blended with this 

 layer. A faint tinge is visible before the completion of the first 

 volution, probably beginning somewhere between the fourth and fifth 

 septa, which gradually extends in breadth and thickness, until it 

 assumes the aspect which distinguishes the adult. The bands on the 

 sides, however, were not observed in the young. 



Valenciennes has fully described these three parts in the adult, dis- 

 tinguishing the internal and external layers, and also the outer colored 

 portion of the last, which, however, he described as a distinct layer. 

 He regarded the two last as confined to the exposed sides and ventrum 

 of the adult, and for this reason attributed their production to the 

 ventral arms of the animal. I am at a loss, however, to account for 

 their imbricated structure, precisely similar to the internal layer, or for 

 the presence of these layers upon the dorsal side of the young, unless 

 they have been deposited from the interior by the edge of the mantle, 

 as were the zones of the internal layer. The replacement of the 

 mantle-edge by the hood when the first revolution was completed, as 

 shown by the appearance of the black deposit, would, if this view were 

 accepted, account for the very slight traces of the external layer left 

 on the dorsal side after the hood came in contact with the apex of the 

 first whorl. A fine specimen of Nautilus Pompilius in the possession 

 of the Museum, with the contained animal in an excellent state of 

 preservation, has been examined, and the structure of the mantle-edge 

 confirms the view here taken. The entire mantle-border is thickened, 

 as described by previous authors, but the edge on the sides and ventrum 

 has a somewhat different structure from that of the dorsum. The edge 

 is tumid and divided, as in the Lamellibranchiata, into lips, containing 

 in the channel between them a brownish substance, probably the re- 



