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 papilliform, This form, how- 

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

 The dimensions of the specimens figured are as follows : 



Var. brunnea Hald. 



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

 this form. 



II. AncyJus tardus Say (1840). PI. I, figs. 11, 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 AVabash. But two dimensions are given, 

 length 215., breadth y L in. (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 

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

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

 terior margin is somewhat broken. Its present dimensions are 

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

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

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



Adams (Thompson's Hist, of Vt., 1842), was next to recognize 

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

 the apex and shape of the aperture. His dimensions (6.25 x 4 x 3.25 

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

 very much higher than the type. 



