74 BULLETIN OF THE 



the convexity of the central portion of the scar was formed, the 

 cicatrix itself being left vacant for the passage of the animal. 



The abrupt and broken character of the border of the external 

 layer on the edge of the cicatrix, and' its crenulated aspect indicate that 

 here is the true location of the former junction of the ovisac and the 

 shell of the first whorl. When we consider how narrow and vertical 

 are the apertures of some of the arcuate Nautiloids of the Silurian 

 epoch, and how closely they approximate to the simplicity of the outline 

 of the cicatrix, this view acquires additional probability, and it seems 

 to be the only one which can reconcile the continuity of the external 

 layer and its markings over this region. 



It still remains difficult, however, to account for the passage of the 

 large body of the embryo through the narrow aperture thus made, 

 and future investigations upon the embryology of Nautilus are much 

 needed, in order to settle this interesting question, as well as the true 

 affinities of the form and structure of the embryo. 



The cicatrix occupies, as has been described, the true apex of the 

 whorl, as determined by the structure of the shell, but only the lower 

 end, which is curved dorsally, occupies the actual apex; the remainder 

 runs along what appears to be the inner or dorsal side, though this 

 really begins higher up at the dorsal border of the cicatrix. 



The absence of the ovisac is due either to its delicacy and the readi- 

 ness with which it could be broken away from its attachment, or to the 

 advance of the mantle, which in course of growth strikes the cicatrix a 

 little inside of the extreme abdominal end, and then bends up over it, 

 and either absorbs or pushes the ovisac away. Whatever may be 

 the ultimate resolution of the question, one fact is very evident : the 

 embryo of Nautilus differs not only in its form, which is a vertical oval, 

 from the Ammonoids, which is a horizontal oval, but in the mode of 

 its passage into the first whorl. 



The whole aperture, or lip, of the ovisac in the Goniatites and 

 Ammonites is united and continuous with the shell of the first whorl, 

 which opens into it at the apex. The siphonal ccecum also has the 

 peculiar pointed cone-like prolongation extending into the ovisac, 

 through the first septum, which shows that the important organ which 

 secreted it differed not only in comparative size, but in shape, and in 

 the earlier period at which it was developed.* The siphonal coecuin 



* Plate III, Figs. 2, 5, 6. 



