PILSBRY: non-marine MOLLUSCA of PATAGONIA. 599 



and third neanic substages of niicrothaimia being superposed upon the 

 adult structure of P. Jiidalgoi. 



Group of P. iheringi. 

 PoTAMOLiTHUS RUSHii Pilsbry. 



(Plate XXXVIII, Figs, i, \a, ib, 4.) 



Potamolithits r/«///V Pilsbry, Nautilus, X, p. 89, Dec., 1896. 



The shell is imperforate, wider than high, biconvex, very solid and 

 strong ; light green, the last half of the last whorl dusky green, the keels 

 rather bright green ; the early whorls being dark reddish-brown. The 

 surface is somewhat glossy, with faint, fine growth-lines and barely per- 

 ceptible spiral lines. The spire is convex, the apex obtuse. Whorls 4, 

 but the first is eroded, leaving a pit, in all the adult shells seen. The 

 whorls are convex, with seam-like sutures. In the latter part of the pe- 

 nultimate whorl the peripheral keel is usually visible at the suture. The 

 last whorl has a very strong peripheral keel, the surface being concave 

 above and below it. Above the concavity the upper surface is convex, 

 the convexity rising into a hump on the back, then disappearing, the last 

 fourth of the whorl being flat. The base has a thick and prominent keel, 

 defining a concave yellowish columellar area. The outer lip has a high, 

 narrow varix at the edge. The aperture is very oblique, short-ovate, 

 nearly circular, with a continuous, black-edged margin. The oblique 

 columella is very broad, with a gutter or concavity near to and parallel 

 with the inner margin. 



Length 4.3, diam. 6.3 mm. 

 " 51 " 6.3 " 



Uruguay River, at Paysandu, Uruguay. Types, No. 69,686, A. N. S. P. 



Development. — The youngest specimens seen have three whorls and a 

 diameter of 3 mm. They have the depressed contour of adults and are 

 strongly carinate peripherally, but the carina is distinctly weaker in front 

 of the mouth, apparently indicating that it begins when the shell has 

 nearly two whorls and a diameter of about a millimeter. At the 3 mm. 

 stage the columella is very broad, semicircular, with a deep excavation and 

 rod-like inner border (PI. XXXVIII, fig. 4). Very late in the neanic stage 

 the basal keel appears, the shell then being about 5 mm. in diameter ; the 

 columellar area being very narrow, at first linear. The rib or convexity 



