MUSEUM OF COMPARATIVE ZOOLOGY. 103 



described by Barrande, in possession of a siphon, or the representative 

 of one in the young, and occupied the entire cavity of the living- 

 chamber; whereas, it subsequently lost this siphon, and built up a 

 portion of the living-chamber with imperfect septa, leaving a cylindrical 

 hollow, which was evidently occupied by the visceral sac, thus plainly 

 retrograding in structure, and undergoing retrograde metamorpbosis, 

 like Orthoceras in the young, and perhaps somewhat similar to Endo- 

 ceras, or others of the Vaginati in the adult. 



THE SHELL. 



The shell of the young Nautilus, at the apex of the first whorl, consists 

 of two layers, an imbricated internal nacreous layer, and a layer of 

 denser texture.* The internal layer is at first a single plate deposited 

 at the apex. The zones subsequently secreted overlie this internally 

 on both the dorsal and ventral surfaces, and from this point the imbri- 

 cated structure is maintained throughout. This shows that the internal 

 layer is entirely deposited from within, and probably by the border of 

 the mantle. Whether seen in transverse or longitudinal sections, it 

 presents the crenulated aspect common to nacreous shells. It is con- 

 siderably thicker on the dorsal side, owing to the longer time which 

 the mantle must take in passing a given point on the dorsal or inner 

 side of the spiral. This thickness is increased by the manner in which 

 the septa extend and overlap their borders on the same side, though in 

 the figure this is not shown, because the cut extended through the side 

 of the dorsal lobe. Subsequently, as the whorl grows larger, this 

 difference between the absolute thickness of the shell decreases, and in 

 the adult they are about the same on either side. 



The external layer is quite thin on both surfaces in the young, but 

 becomes also rapidly thicker on the dorsal surface, and assumes a 

 denser and more opaque structure than the interior. A break in this 

 layer on the dorsal surface just before it completed the first revolution, 

 and came in contact with the apex,f shows that this layer was probably 

 also deposited by the border of the mantle. At this point the layer is 



* Plate IV, Fig. 4. 



t The break is marked by a very faint line in the figure, near the angle of the 

 umbilical perforation. This has accidentally been made too slight in the drawing; 

 the break is much thicker and more decided. 



