PILSBRY: NON-MARINE MOLLUSCA OF PATAGONIA. 593 



peripheral keel and a small keel near the suture, the surface flat between 

 the keels. The base is a little convex. The umbilical area is slightly 

 excavated, but not bounded by a ridge or keel. The suture descends 

 slowly in its last third of a whorl, and then rapidly at the aperture. The 

 aperture is very oblique, shortly ovate, flesh-tinted inside. Peristome 

 blunt, not expanded. Columella concave, rather narrow, its face slightly 

 grooved. Parietal wall rather heavy-calloused. 



Length 4.2, diam. 4.1 mm. 



Paysandu, Uruguay. Dr. Florentine Filippone. 



This species closely resembles P. microthauma, from which it differs 

 by the entire absence of a varix behind the outer lip ; the coloration is 

 also different, but the type is probably abnormally dark, being an old shell 

 which has lost part of the cuticle. 



POTAMOLITHUS TRICOSTATUS (Brot). 

 (Plate XL, Figs. 6, 6a, 66, 7.) 



Lithoglyphus tricostatus Brot, Journal de Conchyl., XV, 1867, p. 68, pi. i, 



fig. 4 (Uruguay River, Province of Entrerios). 



The shell is trochiform, solid and strong ; olive-colored, profusely 

 marked with irregular or zigzag buff spots. The surface is glossy when 

 clean, with the usual weak growth-lines and minute spiral striae. Spire 

 short, convexly conoidal. Whorls 4^ , the first three convex. A ridge 

 then gradually appears below the suture, increasing to a strong carina on 

 the back of the last whorl. The last whorl has also a thick, strong keel 

 defining the base, and a short keel, chiefly dorsal, above the middle of the 

 slightly convex surface between the two keels. The base is flattened, but 

 a little convex ; a small columellar area is usually distinctly differentiated. 

 The last whorl expands slightly at its termination, and then contracts. 

 The aperture is very oblique, rounded-ovate, bluish-white within. The 

 peristome is continuous, black-edged. The columella is narrow and con- 

 cave ; and, with the parietal wall, is moderately calloused. 



Length 5, diam. 5 mm. 

 " 5-3 " 4-8 " 

 4-7 " 3-9 " 



Uruguay River, at Paysandu, Uruguay. 



Development. --The youngest shells seen have nearly 4 whorls. The 



