54 



BULLETIN 111, UNITED STATES NATIONAL MUSEUM. 



in D. ceratum; the relatively short smooth portion of the shell; the 

 shorter length but equal diameter. 



Geographically its range is more northerly and its depth zone less. 

 Its characters are remarkably constant. There is no complete tip 

 preserved among the many lots of the museum collection. 



The following are the museum records: 



KEY TO THE GROUP OF DENTALIUM BARTLETTI. 



Primary riblets 18. 



Section slightly flattened Dentalium bartletti, page 55. 



Primary riblets l(i. 



Section round D. tubidatum, page 56. 



