50 UNIVALVES. ARGONAUTA. 



face, erects its arms, and throws out a membrane between 

 them, by which means it is driven forward, like a vessel 

 under sail : two of the arms it hangs over the shell, which 

 serve as oars and rudder. The Mediterranean often has 

 whole fleets of these diminutive navigators floating on 

 its calm surface. They are, however, difficult to catch, as 

 they are astonishingly expeditious in sinking, whenever 

 they are approached too nearly. The color of these 

 shells is mostly blueish, or yellowish-white, having the 

 keel often tinged with a brownish hue. As to size, the 

 Argonautse differ greatly ; the Argo, for instance, will 

 often grow to ten or twelve inches in width ; whereas the 

 Cornu and Arctica seldom exceed three or four lines in 

 diameter. 



The Mediterranean and Indian seas produce some va- 

 rieties; others are from the Cape of Good Hope, and 

 some inhabit the Northern and Greenland seas. 



None of this genus have hitherto been found fossil. 



They are supposed to have derived their name from the 

 i\rgoiiautse. 



ARGONAUTA. 



Argo Cornu. 



Vitreus. Arctica. 



Cymbium 



