122 THE NAUTILUS. 



7. Pupa montanella Ckll., 1889. 



8. Pupa curvidens var. gracilis Sterki, 1890. 



9. Pupa pentodon for f. curta Sterki, 1894. 



The original descriptions of these forms are given below, most of 

 them in full, while of others (nos. 4-6) only the important parts 

 are quoted. 



Having examined many hundreds of specimens from all parts of the 

 range of the species in question, it appears that there is intergrada- 

 tion throughout the series in a small proportion of specimens ; but 

 the vast majority of specimens fall into two groups which may be 

 given the nominal rank of species, the proposed classification standing 

 as follows : 



B. pentodon (Say). Synonyms, cincinnatiensis, curta, montanella, 

 " Form " curvidens, \nc\ud\ngjloridana. 



Var. gracilis St. 



B. tappaniana (C. B. Ad.). Synonyms, tappiana Ph., pentodon 

 auct. 



NOTE. The figures are all drawn to one scale (X 13.8), and any may be 

 accurately measured by plotting on paper a scale with the lines 13.8 mm. 

 apart ; each space then representing 1 mm. Thus fig. 1 is 1.8 mm. long. 



BIFIDARIA PENTODON (Say). Figures 1 to 41. Fig. 1 is typical. 



The shell is small, varying from conic to subcylindric in shape. 

 Teeth typically 5, of which the parietal, columellar and lower pala- 

 tal are larger and compressed, the lower palatal entering a little more 

 deeply than its fellows in the outer margin ; but often subcolumellar, 

 basal, and interpalatal denticles are developed. Figs. 1 to 8 are 

 from Maine specimens, showing the ordinary variations. The de- 

 velopment of a crest behind the lip varies a good deal, from weak to very 

 strong, with all intergrades. P. cincinnatiensis (fig. 35, a cotype) is 

 a small form of pentodon. 



The increase in number of accessory denticles or teeth culminates 

 in the form named curvidens, represented in figs. 9 to 15, also 3, 6, 

 32 and perhaps some others, as there is absolutely no line to be drawn 

 between pentodon and curvidens. The number of teeth varies from 

 5 to 9 in perfectly adult shells of the same gathering from one 

 spot. The development of an infraparietal denticle is also extremely 

 variable in many lots, as in that from Buckfield, Me., figs. 1 to 4, 8. 

 P. ftoridana (fig. 9, a cotype from the author) is absolutely identical 

 \\ ith curvidens. 



