THE NAUTILUS. 29 



founded on some distinct and constant characters, and reduced to 

 the minimum. 



The shell described by Say is symmetrical, of fine texture, smooth 

 and shining surface, lines of growth scarcely visible even with a 

 lens, whorls four, convex, sutures impressed, spire elevated and 

 acute, and the inner wall of the aperture straight. It is an annuan, 

 rarely taking a second growth, and when it does the growth appears 

 to be an addition to a mature shell. 



Distinguished by its symmetrical form, smooth and shining sur- 

 face, convex whorls, impressed sutures, elevated and acute spire 

 and straight or perpendicular axis, thus forming a straight inner 

 wall to the aperture. 



Physa philippi Ku'ster, is an inflated variety bearing all the char- 

 acteristics of the type, except that it is inflated and the columella is 

 slightly curved. This is the first inflated variety described, and 

 should cover all inflated forms. 



Physa plicata DeKay. This form is admitted a species by 

 Tryon, but Binney, Pilsbry, and others, place it amongst the 

 synonyms of this species, where it evidently belongs, unless it is 

 raised to a variety. It being the only elongated form of this species 

 ever described, it may well be considered a variety to which all 

 elongated forms should be referred. 



Physa heterostropha alba, n. v. Shell ovate, symmetrical, not 

 inflated, porcelain white, whorls four, convex, sutures impressed, 

 spire elevated and acute, surface smooth and shining, columella 

 slightly curved. Diameter § the length. This beautiful little shell 

 was sent to me by Mr. Albert Baily, of Capachet, N. Y., he having 

 taken it from Cedar Lake near that place. It is distinguished by 

 its color. 



I have recently seen in Mr. Bryant Walker's collection some 

 very fine forms of this species that seem to be of frequent occurrence 

 in Northern Michigan. They are slightly more elongated than the 

 type, and are marked with a dark-brown apex. This marking does 

 not appear in any of the eastern forms, so far as I know, but it is 

 not sufficient variation to found a variety upon. 



Further forms are : Physa striata Menke, P. charpentieri Kiister, 

 P. fragilis Mighels, P. lata Tryon, P. primeana Tryon, and many 

 other slight variations which have not been named. 



This species extends from the Potomac and Ohio rivers north and 



