suBCLASS II DIBRANCHIATA 689 



are initiated in tlie Lias. From wliat grouj) Dibranchiates are descended, whether 

 from tlie Tetrabranchiates or from primitive naked ancestors, \ve bave at present no 

 certain iiieaiis for deterniining. They appear suddenly in a liigh state of develop- 

 ment ; biit a still more remai'kable fact is the swift culmination and decline of the 

 group of Belemnoids. In contrast to the small number of forms met witli in the Trias, 

 we find even in the Lias, as well as other divisions of the Jura and Lower Cretaceoiis, 

 a rieh and varied Belemnite fauna. At the close of the Cretaceous only two genera, 

 Belemnitella and Äctinocamax, persist in relatively large niimbers, and althoiigh a 

 few antiquated relics of the same stock continue into the Eocene, their rarity 

 demonstrates waning vitality. The sola living representative of Belemnoids is the 

 genus Spirula. 



In all probability the Sepioidea are descended from Belemnoids. Belosepia of the 

 Tertiary has tolerably distinct indications of a phragmacone, but in Sepia proper the 

 septation has become vestigial. Jurassic Chondrophoridae approximate closely to 

 Recent squids and cnttle-fishes. All the evidenee at our disposal jiistifies the conclusion 

 that Mesozoic Sej^ioids possessed an essentially similar Organisation to that of Recent 

 forms, 



[For certain changes introduced in the present treatment of Dibranchiate Cephalopods, 

 as compared witli the original German edition, the Editor alone is responsible,] 



VOL. 1 2 Y 



