THE NAUTILUS. 



29 



turned from the median line, and steep, nearly straight lateral slopes. 

 The anterior slope is usually more decidedly convex than in rivulariSy 

 and the posterior slope, though frequently quite concave, is shorter,^ 

 more direct and less oblique than in that species. The variations in 

 contour leading towards rivtdaris are shown by figs. 11, 16, 18, 20 

 and 22 on plate I. An unusually narrow form (pi. II., fig. 14) 

 occurred sparingly among the material supplied by Dr. Sterki. 

 The dimensions of the specimens figured are as follows : 



Plate I., fig. 



Plate II., fi 



Figs. 

 Figs. 

 Figs. 

 Figs. 

 Figs. 

 Figs. 



EXPLANATION OF THE TLATES. 



All the figures are drawn on the same scale. 



Plate I. 

 1-5 A. rivularis Say. Erie Canal, N. Y. 



6-8 " " " 



9-10 " " " 



11-12 A. tardus Say. 



13-15 A. rivularis Say. 



16-23 A. tardus Say. 



Lincoln, Neb. 



Lamberton Creek, Kent Co., Mich. 



