PILSBRY: NON-MARINE MOLLUSCA OF PATAGONIA. 581 



its face concave, throughout the neanic stage. Figs, n, na represent a 

 young shell 3.3 mm. in diameter. 



P. biiscJiii is related to P. agapetus, but it attains a larger size, and dif- 

 fers in the coloration and angular periphery ; yet there are some imma- 

 ture specimens in the lot from San Gabriel's Island, which approach very 

 near to P. agapetus. It differs from P. tapidum and P. tricostatus by its 

 wide columella at all stages of growth. It is a very abundant snail at 

 San Gabriel's Island, on the northern shore of the Rio de La Plata. 



The figures on Plate XL represent the least angular forms of the 

 species from San Gabriel's Island. The type was a carinate shell, such 

 as that represented in Plate XLI<5, fig. 3, from Fray Bentos, on the Uru- 

 guay River. The ridge below the suture on the back is usually incon- 

 spicuous. Most of the specimens from San Gabriel's Island are similar 

 to those figured on Plate XL, or somewhat more angular, but less so than 

 shells from the mainland. They also have the ridges or sulcus on the 

 back more strongly developed in some examples. 



POTAMOLITHUS CONICUS (Brot). 

 (Plate XL, Figs. 8, 9, 9 a.) 



Lithoglyphus conicus Brot, Journal de Conchyliologie, XV, 1867, p. 69, 



pi. i, fig. 5 (Uruguay River in the Province of Entrerios). 

 The shell is high-trochiform, rather straightly conic, solid, green or 

 brownish-olive, variegated with pale green or yellow zigzag streaks. The 

 smoothish surface is rather glossy, with the usual fine growth-lines. The 

 conic spire is longer than in related species, and often eroded at the apex. 

 Whorls nearly 4^ , convex. The last whorl slopes steeply and with little 

 convexity to the carinate periphery, which is basal in position. On the 

 back there is sometimes a very weak ridge below the suture and parallel 

 to it, but this is usually wanting. The base is flat, and there is a rather 

 wide crescentic, concave columellar area defined by a sharp angle. There 

 is no varix at the lip. The aperture is very oblique, symmetrically ovate, 

 in fully adult shells is contracted somewhat, being filled in above ; with 

 a continuous, black-edged peristome. The columella is concave and 

 narrow. 



Length 4.7, diam. 3.7 mm. 



3-9 " 3-3 " 



