G. GENERAL NATURAL HISTORY AND ZOOLOGY. 335 



hard parts in the latter, the nidamentary gland is situated 

 on the siphonal side of the outer chamber of the shell, above 

 the adductor muscle^ and outside of the ring of adherence, 

 which last leaves an impression in the shell. In the ammo- 

 nites of the Soleuhofen limestones (the tranquillity of the 

 deposition of the sediments having favored preservation), 

 similar impressions have been found ; they begin at the mar- 

 gin of the aperture, toward the middle of the sides, follow 

 the spiral of the shell backward toward the septum, and final- 

 ly bend toward the siphonal wall; the Aptychus is above 

 and outside of this impression (le., nearer the siphonal side), 

 as is the nidamentary gland in Nautilus, and this position is 

 in the strongest degree suggestive of the nature of the organ- 

 ism in question, that is, that it serves as covering the nida- 

 mentary gland. 



Professor Waagen, assuming as proved this relation, avails 

 himself of the presence or absence (for it is not present in all 

 Ammonites) of the Aptychus^ and its several modifications, 

 for the distribution of the Ammoniticlce into sections,, and 

 subordinates the genera admitted by late naturalists in the 

 following manner : 



A. Nidamentary gland, without solid integument (z.e., with- 

 out Aptychus): 



Phylloceras Suess, Lytoceras Suess, Arcestes Suess, Pinaco- 

 ceros Mojsis, Trochyceras Laube. 



B. Nidamentary gland, with a solid integument (^. e., with 

 Aptychus). 



I. Gland simple, not divided : 



1. Integument horny {Anaptyc7iiis). ArietitesWaag., ^go- 

 ceras Waag., Amaltheus Montf. 



2. Integument calcareous. Aptychus numida, Coq. Shell 

 unknown (Stdetes?). 



II. Gland double, with the Ap)tychiis calcareous : 



\. Aptychus possessing furrows on the external side: 



Harpoceras Waag.^CEkotraustes Waag., Oppelia Waag., Ha- 



plocercis TAXX.^'iScaphitesV^xk. 



2. Aptychus thin, granulated externally : S)tephanoceras 

 \^^2.<g.^ Perisphinctes Wii2(g.,'iPeltoceras Waag., Cosmoceras 



Waag. 



3. Aptychus thick, smooth, and punctated externally. Si- 

 moceras Zitt., Asjyidoceras Zitt. 



