106 GENERA OF SHELLS. 



NAUTILUS. Shell discoidal, spiral, multilo- 

 cular ; parietes simple ; whorls contiguous, the 

 last enveloping the others. Chambers numer- 

 ous. Septa transverse, concave without ; disc 

 perforated by a tube ; margins quite simple. 



N. pompilius, umbilicatus. 



AMMONEATA. 



Septa sinuous, loled and indented at the circumference, united 

 at the inner surface of the shell and articulating with it 

 by means of indented sutures. 



AMMONITES. Shell discoidal, spiral, whorls 

 contiguous, and all of them visible ; the inte- 

 rior parietes articulated by sinuous sutures. 

 Septa transverse, lobed and indented at the 

 circumference ; their discs without any siphon, 

 but perforated by a sort of marginal tube. 



The ammonites differ essentially from the nautili 

 by the sinuous sutures of the internal parietes and by 

 the similarly sinuous form of the septa ; from the or- 

 bulites, by all the whorls being distinctly visible. They 

 are only found in a fossil state. 



ORBULITES. Shell sub-discoidal, spiral, 

 whorls contiguous, the last enveloping the 

 rest ; internal parietes articulated by sinuous 

 sutures. Septa transverse, lobed at the cir- 

 cumference, and perforated by a marginal 

 tube. 



AMMONOCERATITES. Shell horn shaped, 



