MUSEUM OF COMPARATIVE ZOOLOGY. 91 



species, which have similar cone-like young, have also similar septa. 

 Nautilus Koninckii * at least, has septa, which differ from the more 

 advanced stages of the young of Nautilus Bohemicus, possessing a 

 faint dorsal cell, or rather subangular outline to the dorsal side of the 

 septum. The suture of the first septum was observed upon one side, 

 though obscured on the other, and this evidently possesses decided, 

 though very shallow ventral and dorsal cells, with correspondingly slight 

 lateral lobes. The septa were elliptical in outline, with the dorso- 

 ventral axis longest, as in the Silurian Nautili, though they speedily 

 reverse this in course of growth, the transverse axis becoming the 

 longest in the adult. The dorsal lobe appears first in Nautilus Bar- 

 randei, Hauer,f or at least has not been described or seen in any 

 species occurring earlier, though it is a characteristic not likely to 

 escape observation, and was well known to Barrande, Sandberger, and 

 others who have worked up the Nautili of the earlier formations. The 

 developmental history of the septa of the Jurassic species shows a 

 decided acceleration in the septal characteristics. Saemann's original 

 specimen, when thoroughly cleansed,! showed that the outline of the 

 first septum was an ellipse, slightly flattened on the ventral side, cor- 

 respondingly with the flatness of the external outline, and nearly 

 parallel with the area of the cicatrix.§ Sections of two different 

 specimens of Nautilus lineatus show the same characteristics. The 

 first septum of Saemann's cast has an entire cell on the dorsal side, as 

 in the young of Nautilus Koninckii of the Carboniferous, though in 

 accordance with the shape of the dorsum it is much broader, and this 

 cell is still more prominent in the second septum, and broken by a 

 dorsal lobe. The second septum crosses the central axis of the spiral 

 at a different angle from the first septum, and is not, therefore, parallel 

 with the area of the cicatrix. The dorsal lobe is very distinct in the 

 third septum, and is supplemented by a decided though shallow inflec- 

 tion of the large dorsal cell forming a shallow supplementary lobe, such 

 as has been already described in Nautilus Pompilus. The concavity 

 of the dorsum becomes apparent between this and the second septum. 



* Plate IV, Figs. 7, 9. 



t Haidinger's Naturw. Abhandl. Ceph. von Hallstadt. By Hauer. Vol. III. 



J It was covered with a thick coating of iron-rust, which obscured the sutures, and 

 entirely concealed the first septum, which was consequently omitted in Saemann's 

 figure of this specimen. 



§ Plate IV, Figs. 5, C. 



