DIBRANCHIATE CEPHALOPODS. 



89 



±.<^- 



V? 





a 



symmetrical, involuted shell (fig. 474), which is secreted 1:ty 

 the webbed extremities of the two dorsal arms, and is not 

 attached in any way to 

 the body of the animal. 

 Male much smaller than 

 the female, shell - less. 

 This family includes only 

 the single genus Argo- 

 nauta (the Paper Nauti- 

 lus). One or two species 

 of Argonaut have been 

 discovered in the Plio- 

 cene Tertiary. 



FaM. 2, OCTOPODID^. 



— Shell internal, rudi- 

 mentary, represented by 

 two short styles encysted 

 in the substance of the 

 mantle. This family in- 

 cludes the living Poulpes 

 and their allies, but has 

 no fossil representatives. 



Section B. Decapoda. 

 — The Cuttle-fishes of 

 this section have eight 



" arms " and two additional " tentacles," which are much 

 longer than the true arms, and carry suckers on their ex- 

 tremities only, which are expanded and club-shaped. The 

 suckers are pedunculated, the body is furnished with lateral 

 fins, and the shell is always internal. 



Pam. 3. Teuthid^e. — Shell consisting of an internal horny 

 " pen " or " gladius," composed of a central shaft and two 

 lateral wings. Several of such pens may exist in a single 

 indi^ddual, packed one behind the other. Pins mostly ter- 

 minal and angular. This family comprises the living Cala- 

 maries and Squids, and the following fossil genera have been 

 founded upon " pens " which have been discovered in various 

 Secondary deposits : — 



a. Teudoijsis. — Pen lanceolate, produced in front, dilated 



Fig. 474. — ArgonoAita c.rrjo, the "Paper Nau- 

 tilus," female. The animal is re]iresented in 

 its shell, but the webbed dor.sal arms are separ- 

 ated from the shell, which they ordinarily em- 

 brace. 



