THE NAUTILUS. 21 



any person who will hold strictly to the distinctive characters 

 mentioned. But unfortunately Mr. Binney has placed P. fontana, 

 having only three whorls, P. plicata, having /onr to five whorls, and 

 two other species having five whorls each, and two species having 

 flattened whorls, in the synonymy of this sjiecies, which has four 

 convex whorls. There never can be a systematic classification of 

 species until we cease to mix forms in this manner. The number 

 and form of the whorls are the most reliable characters upon which 

 a species can be based. It is one of the laws of nature that every 

 animal, every bird and every insect, of the same species, builds its 

 house in the same form. So the structures of every snail shell of 

 the same species must be substantially the same as to texture, and 

 number, and form of whorls. They may vary in color, height of 

 spire, form of aperture and other characters that may be affected by 

 accident or environment, but as to the structural form of its own 

 skeleton it must follow its progenitors. So I conclude that every 

 adult member of the same species must have the same number of 

 whorls. 



Coming back again to the main question. What is Physa hetero- 

 strophaSa.jl It is a shell subovate in form, having four whorls, 

 spire whorls somewhat convex, sutures impressed, spire elevated, ter- 

 minating in an acute apex, aperture large and oval, but not inflated. 

 The spire whorls are more convex than in P. gyrina Say and not so 

 much as in P. showalterl Lea or P. halei Lea. It holds an inter- 

 mediate place between the flattened and most convex whorls. It is 

 never cylindrical, and the type is not inflated nor ventricose. 

 Varieties may be slightly inflated but never so much as to resemble 

 P. vinosa Gld. or P. sayii Tappan, which are frequently taken for 

 this species. 



If I am not correct, will some person who has seen the type speci- 

 men, revise and correct the description herem given ? 



NOTES ON THE NORTH AMERICAN SPECIES OF SUCCINEA. 



BY T. D. A. COCKERELL 



For some time past I have been accumulating notes on the North 

 American Saccineie, hoping to be able to classify them more pre- 



