SUB-CLASS II 



DIBRANCHIATA 



broadly rounded posteriorly, pointed in front, traversed by a median longitudinal 

 keel. Upper Lias of Wiirteinberg. 



Teuthopsis, DesL Lias. Celaeno, Mtinst. Up]. T 



Jura. Phylloteuthis, Meek and Hayden ; Actinosepia, rf ^*G 



Whiteaves. Cretaceous. 



Plesioteuthis, Wagner (Fig. 1259). Prooetracum v. -i y ' 

 thin, long, narrow, lanceolate, pointed posteriorly, rounded t 

 in front, with a median longitudinal keel and a raised 

 line along each of the lateral edges. Very abundant in 

 Lithographic Slates, and impressions of the body and head 

 not uncommon. Also found in Cretaceous of Maestricht 

 and Syria. 



Sub-Order C. OCTOPODA. 



Body without internal shell, and only the female of 

 Argonauta secreting a single-chambered external shell. The 

 two tentacles are not present, and the eight arms bear sessile 

 suckers without horny rims. Eye relatively small, without 

 sphincter-like lid. Body short and rounded, usually without 

 Jin-like appendages. 



The majority of genera belonging here are naked and 

 therefore without fossil representatives. The small male 

 of Argonauta, Linn., is without a shell, but the large 

 female bears a delicate, boat-shaped, spiral shell which is 

 secreted partly by the mantle, and partly by two fin-like 

 expansions of the dorsal arms. Outer surface of shell 

 ornamented by folds and tubercles, and two nodose ventral 

 keels are present. Late Tertiary and Kecent. 



Fio. 1259. 

 Plesioteuthis proco, RUi 



Vertical Range of the Dibranchiata. 



As compared with Tetrabranchiates, the Dibranchiata 

 are of subordinate geological importance. Their entire 

 organisation renders them less well adapted for preserva- 

 tion in the fossil state, and accordingly we shall never be 

 able to form even an approximate idea of their importance 



in their contemporaneous faunae. The earliest representa- rusiouutms pnsca, Kupp. up. 



tive of Belemnoidea appears in the Trias (Aulacoceras), Lithographic Slates Eichs'uidt! 



, ,, .7 ...... . T . _. . ' 7 A, Impression of annual showing 



and the Sepioidea are initiated in the Lias. From what arms and ink-bag. B, Shell, V* 



group Dibranchiates are descended, whether from the 



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

 means for determining. They appear suddenly in a high state of development ; but a 

 still more remarkable fact is the swift culmination and decline of the group of 

 Belemnoids. In place of the small number 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 rich 

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

 Belemnitella and Actinocamax, persist in relatively large numbers, and although a few 

 antiquated relics of the same stock hold over into the Eocene, their very rarity 

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

 genus Spirula. 



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

 Tertiary has tolerably distinct indications of a phragmocone, but in Sepia proper this 



