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great interest. They occur in all formations, from tte 
Cambrian to the latest tertiaries. Such genera as Orthoceras 
and ISTautilus appear in the Cambrian and Silurian strata, 
and the latter genus still survives. Argonauta is found 
fossil in the Pliocene strata only. The Dibranchiata are of 
comparatively recent origin, occurring for the first time in 
the early secondary rocks. The families Octopodidae and 
Spirulidaj have no representatives in the rocky strata. 
The following tables will serve to illustrate the geological 
distribution of the families and genera of recent and extinct 
Cephalopoda : — 
Distribution of Characteristic Genera. 
1. Pliocene. 
Argonauta. 
2. Miocene. 
Spirulirostra, Aturia. 
3. Eocene. 
Beloptera. 
4. Cretaceous. 
j Scaphites, Hamites, Ptycboceras, Turrilites, Conoteu- 
\ this, Belemnitella, BacuUna, Toxoceras, Crioceras. 
5. Oolite. 
Leptoteuthis, Nautilus. 
G. Lias. 
Belemnites, Beloteuthis, Geoteuthis, Ammonites. 
7. Trias. 
Ceratites. 
8. Permian. 
9. Carboniferous, 
Goniatites, Nautiloceras. 
10. Devonian. 
Clymenia, Bactrites. 
11. Silurian. 
Actinoceras, Phragmoceras, Trocboceras, Ascoceras. 
12. Cambrian. 
Camaro2eras, Endocera?. 
