

The Nautilus. 



Vol. XIX. MARCH, 1906. No. 11. 



ON BIFIDARIA PENTODON AND ITS ALLIES. 

 BY K. G. VAN'ATTA AND H. A. PII.SBRY. 



The group of B. pentodon has been for many years the subject of 

 much perplexity to conchologists, on account of the great variability 

 of the shells in size, shape, and dentition of the aperture. It is easy 

 to select forms which seem perfectly distinct species, but when large 

 series are examined, there is always a residuum of intermediate 

 specimens which can as well be placed in one as another of the se- 

 lected groups. The difficulty is increased by the fact that practically 

 all of the forms occur over the whole area of the group, though one 

 or another of them may predominate in any given locality. This led 

 Dr. Gould to the conclusion that P. curvidens and tappaniana are 

 forms of pentodon (Boston Journal N. H. IV, 1844, 'p. 35), an opin- 

 ion adopted by Binney, and commonly prevalent until recent years, 

 when Dr. Sterki revived curvidens. It is the purpose of this article 

 to ascertain the exact forms which have been described, and to 

 show by figures the various modifications of form. 



In all the forms the shell is of a white-corneous color and the teeth 

 within the lip stand upon a white callus, in this respect differing 

 from B. pihbryana of the Southwest. 



The following names have been applied to the group : 



1. Vertiyo pentodon Say, 1821. 



2. Pupa curvidens Gld., 1841. 



3. Pupa tappaniana Ward, C. B. Adams, 1842. 



4. Pupa tappiana Ward, Pfr., 1842. 



5. Pupa cincinnatiensis Judge, 1878. 



6. Pupilla Jioridana Dall, 1885. 



