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Vorticialis. Shell discoidal, spiral, multilocular; whorls contiguous, 

 not apparent outwardly; septa transverse, imperforate, not extending from 

 the centre to the periphery; aperture marginal. 



A minute nautilus of Fichtel, Ency. Meth. pi. 470, fig. 1, a. b. c. 



Nummulites. Shell lenticular, thin towards the margins; spire internal, 

 discoidal, multilocular, covered by many thin plates; exterior margin of the 

 whorls folded in two, extending from each side of the shell to the centre, and 

 uniting; chambers very numerous, small, alternate; septa transverse, im- 

 perforate. 



A fossil. 



Nautilus. Shell discoidal, spiral, multilocular, sides simple; whorls 

 contiguous, the last covering the others. Chambers numerous; septa trans- 

 verse, concave from the side of the aperture; disc perforated by a tube, and 

 the margins very simple. 



N. pompilius, N. umbilicatus. 



II. Shell multilocular, the septa indented at the borders. 



AMM ONE AT A.— Five Genera. 



Septa sinuous, lobed, 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; the whorls contiguous, and the wdiole 

 of them apparent; the internal partitions articulated by sinuous sutures; septa 

 transverse, lobed, and indented at the circumference, their discs without a 

 syphon, but pierced by a sort of marginal tube. 



A fossil. 



Orbulites. Shell subdiscoidal, spiral; the whorls contiguous, the last 

 covering the others, and the internal partitions articulated by sinuous sutures; 

 septa transverse, lobed at the circumference, and perforated by a marginal 

 tube. 



A fossil. 



Ammonoceratites. Shell coniform, arched, semicircular; partitions ar- 

 ticulated by sinuous, ramose, indented sutures; septa transverse, sinuous, 



