MUSEUM OF COMPARATIVE ZOOLOGY. 105 



mains of the animal matter which colors the external layer. This 

 brownish matter is friable and not a consistent horny membrane or 

 epidermis. The mantle-edge on the dorsal side, however, is thin and 

 lies posterior to the hood described by Owen and Valenciennes which 

 secretes the black layer. That on the ventrum is much thicker, with 

 tumid border and one channel. Farther down on each side, and cor- 

 responding to the increased thickness of the shell, the border becomes 

 exceedingly thick with two channels, an outer and an inner, each still 

 partly filled with soft matter, evidently unformed shelly excretions, 

 whereas the dorsal border has no such material left in its channel. 



Barrande states that the dark layer of the hood is wanting in the 

 Silurian Nautili, and this accords admirably with the continuity of all 

 the layers on the dorsal side in these species, and also in the slightly 

 involute Carboniferous Nautili, which I have examined. Like the size 

 of the umbilical perforation, and the concavity of the dorsum, the 

 presence or the absence of the dark layer and the extent of the exter- 

 nal layers depend upon the greater or less closeness and increasing 

 constancy of the involution of the whorls. It could not be inferred from 

 this, however, that the fold of the mantle or hood was absent or present 

 in the same proportion. The black deposit simply indicates, so far as 

 we know, the secreting power of the hood, and shows that this organ came 

 in contact with the whorls, but nothing more. The external layer is 

 visible with a common hand lens in Nautilus lineatus of the Jura, and 

 exhibits characteristics similar to those of Nautilus Pompilius. 



Lining all of the chambers, and the exterior of the siphon, is an 

 exceedingly thin membrane, which upon the septa, and the sides of 

 the shell proper, though not upon the siphon, is generally, but not in- 

 variably, connected with an equally thin layer of nacre, and in the desic- 

 cated specimens ; they may be observed peeling off together. Even in 

 Jurassic fossil shells, it is distinct and easily traced, probably owing to 

 its connection with the nacreous deposit described above. 



The same layers are traceable in the ventral and lateral sides of the 

 shell of Goniatites. All the layers * consist of imbricated plates, laid 

 on internally. There is in many specimens a dark-colored layer, 

 equivalent, however, to the external layer of Nautilus, and like that, 

 invested by a layer which corresponds to the smooth, colored layer of 

 Nautili Pompilius. In one specimen, figured on Plate IV., I was for- 



Plate IV, Fig. 11. 



