THE NAUTILUS. 129 



manner as has the specimen No. 8 of this plate. From the 

 illustration it would be taken for an ordinary dextral shell. 



DESCRIPTION. 



In the following description only the characters of interest in 

 this discussion are noted: "Shell sinistral, discoidal, more or 

 less rounded; surface shiny, lines of growth oblique; whorls 

 four, rounded above and below, rather subcarinated; gently and 

 regularly expanding; spire flat or on a level with the general 

 plane of the whorls; periphery rounded, aperture lunate, mouth 

 of the aperture dilated to a great extent forming a bell-shaped 

 expansion; last whorl contracts slightly just before the dilation 

 commences; heavy ridge inside aperture beneath constriction 

 forms narrow throat." 



The last whorl in many cases shows a tendency to turn 

 slightly upwards, the effect of which is accentuated by the 

 rapidly flaring aperture. In the normal type this is so incon- 

 siderable as to be omitted in most descriptions of the species. 

 Gould 1 and Haldeman,' however, mention this feature. The 

 former says : ' ' The whorls enclose each other in a very regular 

 spiral to the last fifth of the outer one, where there is a sudden 

 enlargement and distortion toward the left" (i. e. upward). 

 The latter says: " Remarkable for the deflection and dilatation 

 of the last whorl." The figures accompanying the above show 

 the deflection of the lower edge of the aperture to be not more 

 in any case than one-quarter the height of the body whorl. 

 Binney's figure 184, reprinted by Call 3 and others, shows a 

 similar slight deflection. Dr. Baker's 4 plates show no such 

 deflection, and the writer has many specimens in the collections 

 here in which that feature is very inconsiderable. It appears 

 from the fact that so many descriptions are silent on this point, 



' Gould, Invertebrata of Massachusetts, ed. Binney, p. 493- 



'Haldernan, Monograph of the Fresh-water Univalve Mollusca of the 

 United States, part 7, p. 9. 



'Call, R. E. , A Descriptive Illustrated Catalogue of (he Mollusca of In- 

 diana, p. 410, pi. 8, fig. 12. 



4 Baker, F. C., Mollusca of the Chicago Area, Bull. 3, pt. 2, Natural His- 

 tory Survey. Chicago Academy of Sciences. 



