108 BULLETIN OF THE 



is known to have been, partially at least, deposited by the enveloping 

 arms of the animal. The shell consists of three layers ; the inner and 

 outer layers have a similar structure. These two are composed of 

 minor plates or zones of variable lengths, but these are never im- 

 bricated, they are simply laid one upon another.* The median layer 

 is very irregular when seen in longitudinal section, showing thicker 

 and thinner portions, the thicker being somewhat more opacpue than 

 the thinner parts. In a transverse section, this layer presents a similar 

 aspect, but the structure varies from a smooth, even shade, to a granular 

 aspect. When viewed from above, this layer presents a reticulated 

 aspect, due to the presence of numerous white opaque thread-like lines 

 of growth. These are parallel with the lips of the shell, notwithstand- 

 ing their minor irregularities, and may be, in some instances, followed 

 for a considerable distance across the whorl. Especially when they 

 represent the former edge of some one of the numerous mouths, 

 marking the periodical arrest and renewal of the growth. They 

 bend posteriorly on the abdomen, and anteriorly on the sides, and are 

 covered everywhere by the internal and external layers which possess 

 no such marks of growth. There are no marks of an imbricated 

 structure, and the absence of lines of growth from the external and in- 

 ternal layers, as well as their presence in the median layers, shows also 

 that the structure is entirely distinct everywhere from the shells of 

 either Nautilus, Goniatites, or Ammonites. It is evident that the in- 

 ternal layer is deposited by the body of the animal internally, the 

 median layer by either the mantle-edge, or by the anterior edge of the 

 abdominal arms, externally, in successively thicker or thinner, but more 

 or less irregular zones, and lastly, the external layer probably by the 

 inner side of the expanded portion of the arms. 



* Plate IV, Figs. 12-16. 

 Cambridge, June 5, 1872. 



