248 SYNOPTIC COLLECTION. 



ond septum, forming a long funnel with no septum at the 

 bottom. The siphuncle begins to contract and from this 

 time on is without septa, resembling the contracted septa- 

 less siphuncle of Orthoceras which arises in the same 

 way ; i. e., by the formation of long funnels. 



Fig. 2 represents the nepionic stage of Nautilus as seen 

 from the front. The specimen has been obtained by 

 breaking down a full grown shell. The vertical scar 

 marking the position of the protoconch is distinct, while 

 the circular siphuncle is of large size. Fig. 3 is the same 

 seen from the side. It shows the increase in ornamenta- 

 tion from a comparatively smooth shell. The lines of 

 growth do not bend backward in the middle of the ven- 

 tral side but run nearly straight. 



Fig. 4 shows finely the scar and the area of attachment 

 of the protoconch ; the siphuncle and first few septa are 

 also plainly visible. The adult Nautilus is represented 

 by No. 596, the shell ; and No. 597, a section showing an 

 alcoholic specimen of the fleshy animal within its shell. 



The involute spiral form has been attained and the 

 coiling has taken place in such a way that the ventral 

 convex side is outermost and the concave dorsal side 

 innermost. As the shell coiled, the newly formed, more 

 or less plastic whorl came in contact with, and was pressed 

 by the harder and older preceding whorl. The tighter 

 the coiling the deeper we should say would be the impres- 

 sion. This impressed area in the dorsal side of the shell 

 is known as the impressed zone, and is an important 

 character in both Nautiloids and Ammonoids. 



In the adult Nautilus the septa extend backward only 

 a short distance, giving rise to short funnels in place of 

 the long ones of more generalized forms. These funnels 

 are connected by a porous tube secreted by the siphon, 

 and the two taken together form the siphuncle. This is 

 well seen in the section of the shell (Nos. 592, 597) run- 

 ning from the outermost chamber to the innermost. Its 

 position is near the center of the concave septa. It will 



