92 BULLETIN OF THE 



The lateral lobes are only very faintly marked, as is also the ventral 

 cell. In the next three septa all these features are intensified, but 

 in the sixth and seventh septa a change takes place. The ventral cell 

 becomes flattened, and forms a transition to the slightly concave ventral 

 cell of the eighth septum. After this period the concavity becomes 

 deeper, and the shell, by growth, increases the depth and size of the 

 other lobes and cells which have been described, but otherwise does 

 not materially change them. 



Thus the Jurassic Nautili, with a shallow lobe instead of a cell on the 

 abdomen, -do not possess this characteristic in the young, but have the 

 usual projecting cell of the Silurian Nautili. Their septa also resemble, 

 at the earliest period, those of the same forms when considerably older, 

 but this outline changes in the second septum. The development is also 

 accelerated in another way than by the earlier appearance of the Silurian 

 characteristics. The increased involution, as previously shown in the 

 small size of the umbilical perforation, and tendency of the whorls to 

 spread inward, causes the second septum to advance its outer or ventral 

 edge, and make a sharp angle with the first septum. The antero-poste- 

 rior axis of the whorl, in other words, curves suddenly instead of slowly, 

 as in the Silurian and Carboniferous Nautili, and the first septa are not 

 consequently parallel either with the mouth of the ovisac, as indicated 

 by the area of the cicatrix, or with each other.* The second septum 

 resembles the first, however, in its shallowness, and is a transition to the 

 third, which crosses the whorl at the same angle as the older septa, and 

 also resembles them in its deeper concavity, and the shape of the dorsal 

 cell and lobe. The acceleration of the development shows itself very 

 decidedly in the modern Nautilus. The outline of the first septum of 

 Nautilus Pompilius is broader, but otherwise the suture is identical with 

 that of the third septum of Nautilus atratus, and like that has the same 

 trend as the older septa, and is deeply concave. Its breadth, also, is due 

 to the accelerated rate of growth of the young, as are also the absence of 

 any stages corresponding to the first and second septum of Nautilus 

 atratus. The outline of the area of the cicatrix is very similar to that 

 of the first septum in Nautilus atratus, and this shows that, in all 

 probability, the neck of the ovisac resembled in shape the apex of 

 the first whorl in the Jurassic Nautili. 



* Plate IV, Fig. 10. 



