106 BULLETIN OF THE 



tunate enough to find in section all these parts, where a transient 

 mouth had been formed, and the deposits continued from this break. 

 Here all of these layers are decidedly imbricated ; even the thin exter- 

 nal colored zones show their imbrications in its external portion as 

 zones of growth, evidently due to internal deposition. Keyserling has 

 described and Santlberger figured what they have called the wrinkled 

 layer, lying between the involved ventral and the dorsal side of the shell 

 of the next whorl. This I have not seen, nor succeeded in detecting 

 anything analogous in the specimens I have examined. The figures of 

 Sandberger, however, are known to be very accurate, and the wrinkled 

 layer is plainly shown, limited to the involved portion of the whorl, 

 and not extending outside of these limits. The specimens I have 

 examined with success are all young, and it may bo that this layer is 

 absent from the younger periods of Goniatites. Besides the two layers 

 there is, as in Nautilus, an internal lining, layer, or fossilized membrane, 

 visible in some specimens, and this, with the prolonged edges of the 

 septa, appears to be all that is deposited on the dorsal side. Not only 

 the external layer is wanting on the involving dorsal side, as in Nau- 

 tilus, but the internal also. 



The shell of Ammonites also consists of two layers * besides the in- 

 ternal lining layer, which both here and in Goniatites, may be often 

 seen to pass between the septa and the shell, as it really does also in 

 some instances in Nautilus. f It is evidently due to the mantle, which 

 deposits a film upon the surface whenever it rests for a sufficiently 

 long period in any one chamber of the whorl. I was also unable, in 

 the young, to observe any deposit similar to the wrinkled or black layer 

 of the Nautilus or Goniatite. J Only the internal lining layer of the 

 chambers thickened by the edges of the septa is deposited on the 

 enveloped region or dorsal side as in Goniatites. 



The shell in Ammonites, as in Goniatites, also extends over the 

 ovisac on the abdomen and the sides. This, at first, led me to suppose 

 that here, at least, it was deposited by the arms of the animal, as in 

 Argonauta, and as supposed by Valenciennes, for the external layer in 

 Nautilus. But nothing in the structure of the shell itself confirms this 



* Plate IV, Figs. 1, 2, 3. 

 t Plate II, Fig. 1. 



J The wrinkled layer undoubtedly exists in the adults of several species, especially 

 in Amaltheus margaritatus, as described by Sandberger. 



