THE NAUTILUS. 29 



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

 The anterior slope is usually more decidedly convex than in rivularis, 

 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 rividaris 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 1 1., fig. 



EXPLANATION OF THE PLATES. 



AJl the "figures are drawn on the same scale. 



Plate I. 



Figs. 1-5 A. rividaris Say. Erie Canal, N. Y. 



Figs. 6-8 " Lincoln, Neb. 



Figs. 9-10 " Lamberton Creek, Kent Co., Mich. 



Figs. 11-12 A. tardus Say. " " 



Figs. 13-15 A. rividaris Say. " " 



Figs. 16-23 A. tardus Say. 



Plate II. 



Figs. 1-3 A. tardus Say. Wabash River. 

 Figs. 4-6 " Orono, Me. 



Figs. 7-9 Vermont. 



Figs. 10-11 Silver Spring, Pa. 



