SUBCLASS II DIBRANCHIATA 689 



are initiated in the Lias. From what group Dibranchiates are descended, whether 

 from the Tetrabranchiates or from primitive naked ancestors, we have at present no 

 certain means for determining. Tliey aj^pear suddenly in a high state of develoj)- 

 ment ; but a still more remarkable fact is the swift culmination and decline of the 

 group of Belemnoids. In contrast to the small numbei' of forms met with in the Trias, 

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

 a ]'ich and varied Belemnite fauna. At the close of the Cretaceous only two genera, 

 Belcmnitella and Actinocamax, persist in relatively large numbers, and although a 

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

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

 genus Spirilla. 



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 

 sejjtation has become vestigial. Jurassic Chondrophoridae apj)roximate closely to 

 Recent squids and cuttle-fishes. All the evidence at our disposal justifies the conclusion 

 that Mesozoic Sepioids possessed an essentially similar organisation to that of Recent 

 forms. 



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

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



VOL. 1 2 Y 



