PILSBRY: NON-MARINE MOLLUSCA OF PATAGONIA. 577 



POTAMOLITHUS BISINUATUS OBSOLETUS Pilsbry. 



(Plate XLI, Figs. 3-5.) 



PofanwlitJnts bisimiatus obsoletiis Pilsbry, Nautilus, X, p. 88, Dec, 1896. 

 The shell is more slender than P. bisi/inatHS,2.c\x\.t\y ovate-conic, green, 

 without color bands. The outer and basal margins of the peristome have 

 only shallow sinuses in place of the deep onesof /*. bisiiniatiis ; the upper 

 one is often hardly perceptible, and, when developed, is nearer the upper 

 insertion of the lip than \xv bisimiatus. The inner margin of the peristome 

 is less heavily calloused, and the columellar area is excessively narrow, 

 hardly noticeable. The apices are perfect in the type lot, the shell con- 

 sisting of 5>< whorls. 



Length 4.9, diam. 3.5 mm.; aperture 2.8 mm. long. 

 4.6 " 3.1 " 



Rio de La Plata, at San Gabriel's Island, near Colonia, Uruguay. Also 

 Uruguay River, at Fray Bentos, Uruguay. 



A large series from the first locality shows this form to be constantly 

 unlike P. bisimiatus. It has the characters of an immature stage of the latter. 



POTAMOLITHUS GRACILIS Pilsbry. 

 (Plate XLI, Figs. 8, 8^.) 



PotauioIitJiHS gracilis Pilsbry, Nautilus, X, p. 88, Dec, 1896. 



The shell is imperforate, acutely ovate-conic, solid but not thick, olive- 

 yellow, becoming reddish-brown on the spire, or dull green ; smooth 

 and glossy. The spire is straightly conic, rather acute. Whorls 5^ , 

 moderately convex, the last symmetrically rounded. The aperture is 

 ovate, subvertical. The outer lip is not expanded, acute, the edge sinuous, 

 being retracted slightly at the suture, a trifle sinuated in the middle, and 

 having a distinct rounded sinus at the base. The columella is concave and 

 narrow, the parietal callus thin. There is no differentiated umbilical area. 



Length 4.3, diam. 2.7, length of aperture 2.25 mm. 



Uruguay River at Paysandu. Types collected byW. H. Rush, LT.S.N., 

 July 18, 1892. 



This species is related \.o P. bisimiatus, but it is unlike that species in its 

 narrower contour. A long series has been examined. The green speci- 

 mens predominate. 



