CHAPTER XXI. 



OF THE ARGON AUTJ. GENUS XVIII. 



ARGONAUTA. 



"Animal a Sepia. Shell univalve, spiral, involute, membranaceous, and 

 one-celled." — Linn. 



This genus is very distinct from all the rest. The shells 

 of the few species at present known are very thin, some- 

 what boat-shaped (jj/. 5. /. 53) ; one end is slightly rolled or 

 curved inwards ; the aperture or mouth is somewhat heart- or 

 arrow-shaped ; the outer part or back of the shell is called 

 the keel. 



The animal is nearly allied to the Sepia or Cuttle-fish. This 

 is one of the few shells that swims in the sea, and not unfie- 

 quently on the surface. 



Linnaeus describes only two species, Gmelin has added S, 

 and Dr. Turton none. Total, only 5. 



Lamarck has divided this genus into two, as follow : 



Carinaria. Shell univalve, very thin, like a cone, flattened 



at the sides, the apex terminating in a very small 



involuted spire, the back having a dentated keel ; the 



aperture entire, oval, oblong, contracted towards the 



