1. 



10. 



78 THE NAUTILUS. 



T. metastriata is doubtfully distinct from compacta and is probably, 

 at the most, only a local form peculiar to the Black Warrior river. 

 If this is correct, it has priority over compacta. 



The male of T. othcaloogaensis is unknown. 



Key to the Species of Truncilla. 

 A. Males. 

 [ Shell with a distinct radial furrow in front of the posterior 



ridge 2. 



Shell without a distinct radial furrow in front of the posterior 



ridge 12. 



Shell subquadrate or subtriangular ; radial furrow sub- 



.'ertical 3. 



Shell suboval ; radial furrow oblique 8. 



Shell subcompressed, subquadrate or broadly subtriangular ; 



umbonal region flattened, beaks depressed 4. 



Shell subtriangular ; beaks elevated, umbonal region in- 



[ flated 7. 



(Posterior ridge strongly biangulate biemarginata. 



(Posterior ridge rounded 5. 



Central ridge well developed, forming an obtuse angle in the 

 basal line at its extremity, between which and the pos- 

 terior ridge the base line is nearly straight, radial furrow 



5. \ wide and conspicuous 6. 



Central ridge less developed, not projecting beyond the basal 

 outline, which is scarcely emargiuate in front of the pos- 

 terior ridge, radial furrow subobsolete . . . stewardsonii. 

 \ bhell subquadrate, solid, central ridge subnodulous, radial 



- \ furrow deep foliata. 



\ Shell smaller, subsolid, central ridge smoother, radial furrow 



[ less impressed lewisii. 



f Posterior slope smooth, nacre white personata. 



"(Posterior slope radially striate, nacre usually ^uv^Xe.hayesiana. 



( Posterior slope convex, posterior end below the middle . . 9. 



8. •< Posterior slope nearly straight, posterior end at or above the 



( middle 10. 



Central ridge nodulous perphxa. 



Central ridge smooth propinqua. 



(Post-basal margin emarginate 11. 



\ Post-basal margin squarely subtruncate sulcata. 



Shell short oval, post-basal emargination deep, umbonal 



region greatly inflated sampsoni. 



Shell more elongate, post-basal emargination shallow, um- 

 bonal region less inflated perphxa rangiana. 



Shell smaller, posterior ridge biangulate, basal emargination 

 shallow, beaks prominent deviata. 



{ 



11. < 



