SUB-CLASS II DIBRANCHIATA 603 
broadly rounded posteriorly, pointed in front, traversed by a median longitudinal 
keel. Upper Lias of Wiirtemberg. 
Teuthopsis, Desl Lias. Celaeno, Miinst. Upper 
Jura. Phylloteuthis, Meek and Hayden; Actinosepia, 
Whiteaves. Cretaceous. 
Plesioteuthis, Wagner (Fig. 1259). Proostracum very 
thin, long, narrow, lanceolate, pointed posteriorly, rounded 
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 
sphineter-like lid. Body short and rounded, usually without 
Sin-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 Recent. 



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 Fic. 1259. 
in their contemporaneous faunae. The earliest representa- — pyesioteuthis prisca, Riipp. sp. 
tive of Belemnoidea appears in the Trias (Aulacoceras), Tithographic Slates; Nichstadt, 
ae = Bac , ’ pression of animal showing 
and the Sepioidea are initiated in the Lias. From what arms and ink-bag. 3B, Shell, 1/p. 
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 Actinocamaz, 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 Sepzoidea are descended from Belemnoids, Belosepia of the 
_ Tertiary has tolerably distinct indications of a phragmocone, but in Sepa proper this 

