MUSEUM OF COMPARATIVE ZOOLOGY. 75 



of Nautilus was not formed until after the animal had passed its first 

 stage of growth and occupied the first whorl sufficiently long to build 

 the first true septum. Even then this organ had not the size and im- 

 portance to which it subsequently developed. I have examined the 

 apices of many fossil Nautili without succeeding in finding any suffi- 

 ciently well preserved to show the original condition of the external 

 shell. One fine specimen of Nautilus Koninckii, from Tournay, had 

 apparently a smooth termination ; the longitudinal plications which 

 cover the young shell of the ornamented Carboniferous Nautili reached 

 only a little beyond the second septum. The whorl was here a rapidly 

 increasing cone, the abdomen, however, quite as gibbous as the dorsum, 

 whereas in the adult the latter is the more prominent, the abdomen 

 becoming deeply inflected. The termination of the whorl was very 

 much flattened, so that from the side it had quite a pointed aspect, 

 whereas an abdominal view showed it to be rounded at the extremity.* 



Nautilus Koninckii, it will be remembered, is a Carboniferous species 

 with a very large umbilical perforation. In fact, the whorls do not 

 even touch at first. The tip of the cone is free for some distance 

 before the involution brings the whorls in contact. No marks of a 

 cicatrix were discernible. 



Saemann's original specimen of Nautilus atratus is the finest I have 

 ever seen, and yet this is only a cast. The apex, however, is formed 

 by a cake of iron, which has a rough, lumpy surface, difficult to ac- 

 count for on the supposition that it was the cast of the smooth interior 

 of an unbroken shell.f The area between the first and second 

 septa is smooth, the abdomen flattened, and a faint median depression 

 is noticeable near the suture of the first septum. This, and the second 

 septum, incline towards the umbilicus at a greater angle than any of 

 the succeeding septa, j The point of the external shell is the corner 

 which it makes on the abdominal side, as it passes around the angle of 

 suture. This has been habitually mistaken for the apex, whereas the 

 organic apex is really further inward, and nearly parallel to the first 

 septum, and, in some Nautili, such as Nautilus lineatus Sow., is an 

 almost flattened area apparently on the dorsal side. § Nothing ap- 

 proaching a cicatrix was actually discovered, and yet, besides the general 

 form, which is similar to that of N. Pompilius, the aspect of Saemann's 



* Plate IV, Figs. 7-9. J Plate IV, Fig. 5. 



t Plate tv pig. g. 5 Plate IV, Fig. 10. 



