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by Professor Surss was ably followed up by Dr. W. Waacen,* 
who directed attention to the importance of the study of the — 
Aptychus and Anaptychus, and the presence or absence of this 
body in the Ammonite shell. The observations made by 
Waacen on the Aptychus he described and applied in the 
diagnosis of the genera proposed for several Jurassic species. 
In the following figures the Aptychus carinatus (fig. 3) is seen 
apart from the shell. In figure 1 it is seen lying in the 
dwelling chamber of an Ammonite, and in figure 2 the 
Aptychus is vertical, as if closing the aperture of the shell. 
The following organic features are considered as elements of 
primary importance in studying the Ammonite shell, and their 
combination constitute characters for the diagnosis of the 
generic groups. 
* “Die Formenreihe des Ammonites subradiatus,”’ Benecke’s “ Geogr.- 
Palzont. Beitrage,” Bd. ii, 1869. 
