MUSEUM OF COMPARATIVE ZOOLOGY. 107 



view. The ovishell of Deroceras planicosta, as figured below, * is com- 

 posed of two layers, — the internal lining layer, and an external thicker 

 layer, which is not continuous with any of the layers in the shell of 

 the first whorl. The internal lining layer bends internally, and coats 

 the inner side of the first septum. The external layer is very thin 

 upon the dorsal side, and was perceptible only by the aid of a fif- 

 teenth. The increase in thickness of the external layer is cpiite abrupt 

 upon the abdomen, but not so abrupt as upon the sides, especially at 

 the edges of the embryonal umbilici. f The external layer reaches 

 beyond the neck of the ovisac, overlapping the edges of the layers 

 which compose the apex or beginning of the first whorl. These are 

 evidently the product of the mantle-edge, and show that the external 

 layer of the ovisac must have been secreted from the interior by the 

 body of the embryo. The shell of the whorls consists at first of two 

 layers, besides the internal lining layer of the chambers. J These, 

 unlike the external layer of the ovisac, to which they collectively 

 correspond, are absent from the dorsum. The presence of the external 

 layer on the dorsum, and its extreme thinness there, as compared with 

 what it is upon the abdomen and sides of the ovisac, is a characteristic 

 also of the adult of Nautilus. 



The absence of all the external layers, however, from the dorsal side 

 in Goniatites and Ammonites, appears to begin at the earliest period 

 in the closely involved species. The septa upon that side in the latter, 

 from the first to the sixth inclusive, extend their borders considerably, 

 and build up quite a dense thick layer, which, however, ceases with 

 the seventh septa. § After this, only the lining layer, probably accom- 

 panied by a thin nacreous deposit, is found on the dorsal side. A 

 close examination of the layers showed everywhere indications of arrests 

 of growth, and the formation of more or less permanent mouths, || or 

 zones of growth, always, however, imbricated as in Goniatites and 

 Nautilus. The last formed, beginning inside of the edges of the earlier 

 deposited zones, a structure entirely at variance with the supposition 

 that any portion of the shell was laid on from the exterior. 



Comparisons have also been made with the shell of Argonauta, which 



* See woodcuts on last page of the text. 



+ Plate II, Fig. 3. 



J See last page of the text. 



§ Plate II. Fig. 1, and last page of text. 



|| Plate IV, Fig. 1. 



