82 STUDIES IN GEOLOGY, No. 3 



date threads, of which there are three on the first whorl, the 

 anteriormost the strongest, forming the carina. On the 

 second whorl a fourth faint thread appears above the pos- 

 teriormost of the three mentioned, and on the third whorl an 

 intermediate thread appears behind the carinal thread, as 

 well as a subsidiary thread before the carina. On the fourth 

 whorl the subsidiary threads have grown equal in strength to 

 the others. This sculpture of six equal threads continues 

 over the rest of the shell, with the introduction of faintly 

 visible intermediate threads between the first and second, 

 third and fourth, and fourth and fifth strong threads, and 

 the addition of a faint posterior thread near the suture. The 

 fifth whorl is rounded in outline, the carina having lost prom- 

 inence, and succeeding whorls are more rounded, with the 

 point of maximum diameter at the location of the obsolete 

 carina. 



(b). Neanic whorls strongly carinate, the carina at the 

 anterior third, accentuated by a coarse thread. The first 

 three whorls below the protoconch bear two prominent 

 cordate threads, of which the anterior one, on the carina, is 

 stronger ; on the fourth whorl a faint thread appears on the 

 basal surface before the carina ; this is strengthened on the 

 fifth whorl, where another light thread appears above the 

 posterior of the two original ones. On the sixth whorl 

 there occurs in addition a faint intermediary thread between 

 the original two, which have lost strength, comparatively; 

 on the seventh whorl the threads mentioned approach 

 equality in strength, and a faint posterior thread appears. 

 The characteristic adult sculpture of the species is attained 

 in the eighth whorl through the interposition of a sub- 

 sidiary thread between the fourth and fifth of those named 

 for the preceding whorl. The seventh and eighth whorls 

 are more evenly sculptured and contoured, the carina being 

 obsolescent, and all the threads save the 5th and 7th attaining 

 equality in strength. Length of eight whorls of form (a), 

 20 mm.; of form (b), n mm. 



