THE NAUTILUS. 45 



stricted aperture and umbilicus, which will be found in a large part 

 of them. It is more like P. solida Philippi than any other species. 

 Physa gyrina Say. 



Including the subspecies Physa hildrethiana Lea, oleacea Try on, 

 and albofilata Ancey. 



This species was found in Bowyer Creek, near Council Bluffs, 

 Iowa, in 1821. It is a large, coarse shell, sub-cylindrical in form ; 

 the largest sizes having six whorls. The lip joins the body whorl at 

 an acute angle, forming a loop-shaped aperture ; the penultimate 

 whorl is enlarged, giving the spire a blunt appearance, lines of 

 growth coarse, crossed by microscopic striae, giving the surface a dull 

 appearance. 



Diameter y 8 -^ the length. This species is a tri-annuan. 



Distinguished by its form, coarse lines of growth, loop-shaped 

 aperture, large penultimate whorl and obtuse spire. 



Physa hildrethiana Lea, comprises nearly all the forms found east 

 of the Mississippi river. It is distinguished from the type by being 

 of a lighter texture, finer lines of growth, not so much covered with 

 cross stria-, and having a darker color. Many of these forms are 

 more slender. 



Physa oleacea Tryon. This variety is more cylindrical than 

 either of the other forms, has a very obtuse spire, lines of growth 

 much finer and the cross stria? more thickly covering the surface, 

 giving the shell a smooth, dull appearance. It is generally found 

 with five whorls. The spire is very variable, but otherwise its 

 markings are constant. Some forms are cylindrical with the spire 

 smoothly rounded over, showing no raised apex. This form Prof. 

 Tryon, in the " Monograph of the Fresh-water Univalve Mollusca," 

 placed with the synonyms of P. elliptica Lea. It is sometimes so 

 distinct from all other forms of this species that I should accord it 

 the rank of a species but for the fact that my observations while col- 

 lecting them have clearly disclosed so many proofs that it is only a 

 peculiar and eccentric form of P. gyrina. I have found it inhabit- 

 ing little ponds, with the typical form maintaining family relations, 

 and of all sizes and forms from the cylindrical scarcely raised spire 

 to the elongated and obtuse spire of the typical form. 1 am fortified 

 in this position by Prof. Tryon, to whom I took some samples for 

 identification about a year before his death, and he identified them 

 as a variety of P. gyrina. 



[To be concluded.) 



