40 



LAND AND FRESH-WATER SHELLS OF N. A. 



PART III. 



vex, scalariform, separated by a deep and channelled suture ; aperture 

 ovate acute, expanded at base, very white within, and with a small 

 umbilicus. 



Paludina crassa, Say. 



"We do not agree with Mr. Say in placing this shell among the Paludinx : 

 it has not their essential characters, excepting the lengthening of the 

 spire. In other respects it is more nearly allied to the AmpulUiriso, its 

 form and thickness particularly approaching some of the fossil species of 

 the environs of Paris described below. 



This shell is oval, elongated, acute at the summit, rounded at base, 

 thick, solid, heavy, covered with a brownish, sometimes greenish very thiu 

 epidermis, below which the shell is of an uniform milky white pureuess. 

 The spire is elongated, conic, scalariform, formed by six convex whirls, 

 deeply separated by a canaliculated suture, and, in perfect specimens, 

 marked with delicate transverse striae. The aperture is moderate, not 

 oblique to the shell's axis or rounded or with a continuous peritreme like 

 Paludina, but oval, narrowed above where it also is angular as in most 

 Ampullarise, enlarged below, where it forms a large, not deep sinus, in this 

 point also resembling Ampulluria, but differing from all Paludinse; lastly, 

 the right lip is slightly reflected (recouvrant), which is also characteristic 

 of the genus in which we have placed it. Right lip acute, quickly thicken- 

 ing but with no rim within, sinuose, especially at base, when viewed in 

 profile ; left lip thickened, especially towards the posterior angle of the 

 aperture, and obliquely appressed so as to blend with the columella which 

 is rounded, thick, and reflected, with a small umbilical opening behind 

 it. This shell comes from the Ohio and most of the North American rivers. 

 Length from 45 to 50 mill. {Deshayes.) 



Reeve, /. c, places Paludina ponderosa in the synonymy of 

 Pal. deciaa. It is, indeed, difficult to draw the line between the 

 two. 



