GEOGRAPHIC DISTRIBUTION. XIX 



c'' Length 14-15 mill., nearly 8 times the d\arn.,fisheri, 



p. 36. 

 a'. Shell longitudinally ribbed. 



b. 6-ribbed at apex, increasing to 12, and at aperture with 

 17-24 alternating riblets ; length 27 mill., about 9 times 

 the diam., oerstedi, p. 24. 



6'. Similar, but glossy with finer sculpture and more nu- 

 merous riblets at aperture, var. numerosum, p. 25. 

 b". 12 to 20 sharp riblets at apex, 25-48 at aperture, the 

 interstices wider than ribs, concave ; length 29-65 

 mill., 9 to 15 times the diam., agassizi, p. 26. 

 b'". About 50 riblets and threads ; shell very large, strong 

 and solid ; aperture oblique ; length 90-99 mill., 5 to 

 5 times the diam., megathyris, p. 67. 

 a". Shell with fine, evenly engraved longitudinal striae toward the 

 apex or throughout ; section circular. 



b. Apex with a straight, narrow slit across an obliquely 

 conic, smooth plug ; shell cylindrical ; length 24 mill., 

 15 times the diam., sectum, p. 96. 



b'. Apex with slit on concave side ; shell translucent whit- 

 ish with opaque rings ; length 30 mill., 16 times the 

 diam., inversum, p. 95. 



b". Apex simple ; length 25-30 mill., 10 times the diam. 



semipolitum, p. 91. 



d". Shell without longitudinal sculpture, rounded or oval in sec- 

 tion. 



b. Strongly compressed between convex and concave faces, 



rapidly tapering ; length 9'5 mill., about 4 times the 



diam. brevicornu, p. 125. 



b'. Shell compressed laterally, excessively slender, fi agile, 



salmon tinted ; length 17, greatest diam. 0'8 mill. 



innumerabile, p. 119. 

 b": Shell subcircular in section. 



c.' Well curved, polished, flesh-tinted toward the apex, 

 which is sometimes slit in front and behind ; length 

 45 mill., 12 times the diam. splendidulum, p. 96. 

 c'. Curvature very slight ; thin, white, smooth. 



d. Almost perfectly straight, fragile, excessively 

 slender ; length 31 mill., nearly 20 times the 

 diam., cequatorium, p. 112. 



