360 AXEL A. OLSSON 



c. Concentrics small, threadlike. 



7. Shell small (length 25 mm.), subelliptical, convex and thin. Color a 

 dingy white, the interior yellow or purple. 



S. ■pallida 



d. Concentric elements stronger. 



da. Concentric riblets coarse and regular, not divided or waved. 



8. Obliquely subovate, coarse textured, the posterior side weakly flexed. 

 Color white, narrowly banded and rayed with purple, the umbones 

 yellow. 



S. formosa 



9. Subrectangular, the beaks nearly median, concentric rugae strong and 

 coarse. Posterior zone well marked, subtruncate at the end. 



S. craneana 



10. Shell smaller, obliquely elliptical, the posterior side much shorter. Gen- 

 erally rayed with white, violet or purple, the interior white or variously 

 stained. 



S. venusta 



db. Concentric elements entire but irregular and strongly waved. 



11. Subovate, heavy, the sculpture formed by irregular, broken, ropelike 

 concentrics, Galapagos. 



S. punctata 



12. Trigonal-subovate in shape, thinner, depressed. Sculpture of waved 

 concentrics overrun by vermiculate striae. Posterior side strongly flexed. 



S. tortuosum 



dc. Concentric elements not entire but splitting into scalelike segments 

 on the ends. 



13. Medium size (length 35 mm.), subelHptical, heavy, depressed. Color 

 white mottled with purple. 



S. verrucosa 



dd. Concentric sculpture sharp or rasplike, formed by thin, raised lamellae, 

 their interspaces flat, narrow or wide, smooth or cut by fine, radial 

 striae. 



14. Shell rounded to orbicular in shape. Surface cream-colored, often deeply 

 suffused with red or pink. 



S. rosea-S. tabogensis 



15. Shell longer, elliptical, the beaks submedian and pointed. Color usually 

 a purple or dull lilac, sometimes white, the umbones yellow. 



S. sozverbyiS. jovis 

 Bb. Sculpture including strong, radial elements over the whole disk or 

 confined to one or both ends. 



e. Concentric sculpture reduced to lines of growth only. 



16. Shape obliquely subovate with high, trigonal beaks. There is an ex- 

 cavated lunule and a flattened escutcheon in the left valve. Sculpture 

 formed by low, irregular, radial riblets, the whole surface coarsely 

 granulose. Interior apricot-colored. 



S. pilsbryi 



f. Sculpture with strong concentrics and radials. 



fa. Sculpture strongly cancellated over the whole surface. 



17. Shape obliquely elliptical, the anterior side longer. Color a creamy white 

 and with violet zigzag markings. 



S. pacific a 



18. Shell smaller, ovate, the ends nearly equal. Sculpture rather coarse. 



S. margarita 



