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NAUTILID^. 



I. Partitions of the chambers with simple margins, forming harmonic 



sutures. 



a. Shell spiral. 



b. Whorls discoid. 



c. Last chambers produced. 

 Spirula. 

 Spirolina. 

 cc. Last chambers uniform. 



d. Sides similar, the mouth mesial. 



Nautilus. 

 dd. Sides dissimilar, convex above, flat beneath, the 

 mouth lateral. 

 Rotalia. 

 Lobatula. 

 I^ummzdita. 

 lb. Whorls globular. 

 Vermiculum. 

 Arethusa. 

 Lagenula. 

 aa. Shell produced. 

 Orthocera. 

 Belemnita. 

 Conularia. 



II. Partitions of the chambers with waved margins forming serrated 



sutures. 



a. Spiral. 



b. Spirally discoid. 

 Ammonita. 

 Orbulila. 

 Scaphita. 

 bb. Spirally turrited, 

 Turrilita. 

 I aa. Shell straight or bent. 

 Baculita. 

 Amplexns. 



