80 THE NAUTILUS. 



Posterior ridge inflated, basal expansion rounded, its margin 

 ., i rounded 7. 



Posterior ridge not inflated, basal expansion flattened, its 

 margin subquadrate penita. 



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

 erous, fine, capillary rays brevidens. 



Shell smaller, subtriangular, posterior ridge inflated, rayless. 



7. 



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. 



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



o the marsupial expansion, nacre usually purple . hayesiana. 



I Posterior slope scarcely striate, no median furrow, nacre 



white . othcaloogaensis. 



Marsupial expansion a prolongation of the median ridge, 

 separated from the posterior ridge by a wide emargina- 



tion 10. 



9. 4 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. 



( Marsupial expansion rounded 11. 



{ Marsupial expansion triangular lewisii. 



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. 



f Shell quadrate, rather solid, marsupial expansion extending 



i 9 J beyond the posterior ridge sulcata. 



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

 ing beyond the posterior ridge lenior. 



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

 supial extension thin, shining, and of different texture 



.. J from the rest of the shell 14. 



j 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. 



caps af or mis. 



, Shell with a distinct radial furrow extending to the beaks.16. 

 ' Shell without a distinct radial furrow 17. 



