THE NAUTILUS. 27 



more oblique. Tliis last specification is perhaps a more reliable dis- 

 tinction in doubtful cases than any of the others. Occasionally 

 depressed specimens occur in which both of the side slopes are de- 

 cidedly concave, and the apex quite papilliforra. This form, how- 

 ever, seems to be an individual rather than a racial peculiarity. 

 The dimensions of the specimens figured are as follows : 



No. Length. Breadth. Altitude. 



Var. hrunnea Hald. 



I have been unable to get any information whatever in regard to 

 this form. 



II. Ancylus tari MS..&M.l\3A<'». PL I, fig?- H, 12, 16-23. Pi. 

 II, figs. 1-22. 



Say differentiated this species from his rivularis on three grounds : 



1. The apex inclined backwards, but not laterally. 



2. The straight posterior slope. 



3. The oval aperture, not distinctly narrowed at one end. 



His type came from the Wabash. But two dimensions are given, 

 length 2"^^, breadth jL jn. (3.75 x 2.5 mm.). Binney errs in quot- 

 ing the length as 4.25 mm. He also states that the type is in the 

 collection of the Philadelphia Academy. Among the Ancyli re- 

 ceived from the Academy is a single specimen (No. 58045) labelled 

 " AVabash River, Mrs. L. W. Say," which is undoubtedly the speci- 

 men referred to. As shown by the figure (PI. II, fig. 1), the pos- 

 terior margin is somewliat broken. Its present dimensions are 

 exactly those given by Say (3.75 x 2.5). It is a dead, somewhat 

 worn, siiell, and the defective peritreme might easily have escaped 

 observation. AVlien perfect, it must have been somewhat larger. 



Adams (Thompson's Hist, of V't., 1842), was next to recognize 

 the species, whicli he says differs from rivularis in the position of 

 the apex and shape of the aperture. His dim^jnsions (6.25 x 4 x 3.25 

 mm.) indicate a much larger shell, proportionately a little wider and 

 very niiicli iiigher than the type. 



