90 GENUS XVIII. ARGONAVTA. 



angle of the keel (see Argonauta vitrca, Favanne, t. 7. 

 f. C 3). 

 Argon A uta. Shell univalve, very thin, boat-shaped, the spire 

 rolled into the aperture, the keel of the back double and 

 tuberculated (see Argonauta Argo, f. 53). 



