24 



rolled in the same plane and forming a single chamber (monothalamous, as 

 in the Argonaut), sometimes with the turns rolled in the same plane 

 and over-lapping and divided into many chambers (polythalamous, as in 

 the Nautilus) ; sometimes partly internal and partly external with the 

 turns in the same plane and not in contact (as in the Spirula). 



Arrangement of the Class. 



The recent Cephalopoda are divided into two orders, first, the Dibran- 

 chiata, with two gills, of which there are few fossil representatives, and 

 secondly, the Tetrabranchiata, with four gills, to which the fossil genera 

 of this class mostly belong. 



The former of these (the Dibranchiata) includes the sections of the 

 Octorjoda (with eight arms) and the Decapoda with eight arms and 

 two tentacles) ; and the latter, the Tetrabranchiata, comprises the Nautili, 

 with many arms, and the allied fossil forms, of the Ammonites, Ceratites, 

 Goniatites, Clydonites, etc. 



The sections of the Octopoda and Decapoda are separated into families 

 and genera distinguished by the attachment of the head, the position of 

 the fins, the plan of the arms, the arrangement of the suckers, etc., and 

 may be thus tabulated. 



Dibranchiata 



Octopoda...^ 



fl. Eledoue 

 I 2. Bolitaona 

 Cirroteuthis 

 Octopus 

 Scaeurgus 

 Pinnoctopus 



7. Philonexis 



8. Tremoctopus 



l^III. Argonautidae 9. Argonauta 



I. Octopidae 



II. Philonexidae. 



V, 



fIV. Onychoteuthidae 



V. Chiroteuthidae 



^Decapoda... { 



VI. Gonatidae. 



VIII. Fidenidaa 



IX. 



Tetrabranchiata... 



'10. 



11. 



Enoploteuthis 

 Abralia 



12. 

 13. 



Octopodoteutbis 

 Ancistrocheirus 



^ 14. 



Onychoteuthis 



15. 



Dosidicus 



16. 



Ommastrephes 

 Tbysanoteutbis 



{18. 



Cbiroteutbis 



(19. 



Histioteuthis 



/20. 

 21. 



Loligo 

 Gonatus 



22. 



Eossia 



1 23. 



Sepioteutbis 



24. 



Loligopsis 



/ 25. 

 ) 26. 

 27. 



Sepiola 



Sepioloidea 



Fidenas 



1 28. 



Crancbia 



29. 



Sepia 



30. 



Spirula 



31. 



Nautilus 



