10 



80 THE NAUTILUS. 



r Posterior ridge inflated, basal expansion rounded, its margin 



„ J rounded 7. 



j Posterior ridge not inflated, basal expansion flattened, its 



[ margin subquadrate penita. 



Shell large, subquadrate, disk distinctly flattened, with num- 

 erous, fine, capillary rays hrevidens. 



Shell smaller, subtriangular, posterior ridge inflated, rayless. 



compacta. 

 Shell still smaller, beaks less elevated, greatly inflated pos- 

 teriorly, rayless metastriata. 



Shell very small, subquadrate, marsupial expansion narrow 



and not much inflated modicella. 



r Posterior slope radially striate, a median furrow in front of 

 J the marsupial expansion, nacre usually purple . kayesiana. 

 I Posterior slope scarcely striate, no median furrow, nacre 



j^ white . othcaloogaensis. 



' Marsupial expansion a prolongation of the median ridge, 

 separated from the posterior ridge by a wide emargina- 



tion 10. 



-^ Marsupial expansion occupying and nearly filling the radial 



furrow, not projecting beyond the basal line , . personata. 



Marsupial expansion at posterior base and separated from the 



posterior ridge by a deep sulcus, notching the margin . 12. 



f Marsupial expansion rounded 11. 



( Marsupial expansion triangular lewisii. 



r Shell large, center of disk inflated, marsupial expansion sep- 

 arated from posterior ridge by a deep median furrow. 

 11. \ foliata. 



Shell much smaller, center of disk compressed, no median 



furrow stewardsonii. 



r Shell quadrate, rather solid, marsupial expansion extending 



TO J beyond the posterior ridge ... .* . . . sulcata. 



j Shell smaller, oblong, thin, marsupial expansion not extend- 



[ ing beyond the posterior ridge lenior. 



Entire post-basal region flatly and broadly expanded, mar- 

 supial extension thin, shining, and of different texture 



from the rest of the shell 14. 



Expansion of the post-basal region less extensive, less com- 

 pressed, and of the same color and texture as the rest of 



the shell 15. 



Shell with a distinct radial furrow and nodulous median 



ridge perplexa. 



Shell smaller, smooth, and without a distinct radial furrow. 



capsceformis. 



f Shell with a distinct radial furrow extending to the beaks.lG. 

 I Shell without a distinct radial furrow 17. 



13. ^ 



14 



15 



