580 PATAGONIAN EXPEDITIONS: ZOOLOGY. 



This snail has the texture, color and size of P. agapetus, from which it 

 differs by the longer, Amntcola-Vkz. shape and the small aperture. There 

 is no differentiated umbilical area. 



POTAMOLITHUS BUSCHII (Ffld.). 

 (Plates XL, Figs. 11-14; XLI, Fig. 3.) 



Lithoglyphus buschii Dunker, Frauenfeld, Zoologische Miscellen, V, in 

 Verhandlungen der k. k. zoologisch-botanischen Gesellschaft in 

 Wien, 1865, XV, p. 530, Taf. 11, first two figures. 

 The shell is obliquely trochiform, being flattened below, semiglobose 

 above ; moderately solid ; green or olive-green with irregular buff flecks 

 or zigzag streaks, and usually a dusky-brown band midway between the 

 periphery and suture. Surface with faint growth-lines and indistinct spiral 

 striae. Spire very short, the apex usually reddish. Whorls 4^/2 , convex, 

 the first one or two usually eroded in adult shells. The last whorl is 

 carinate, strongly angular, or with a rounded angle at the basal periphery, 

 very convex above the keel, and usually having a low ridge on the back 

 a short distance below the suture ; there is also, sometimes, a second 

 obtuse ridge below the upper one (fig. 13), much as in P. lapidum super- 

 sulcatus. Base flattened, but slightly convex. There is a well developed 

 concave columellar area bounded by an acute ridge. The aperture is 

 very oblique, ovate, the outer lip without a varix, but built downward 

 somewhat near and at the upper angle, contracting the aperture. The 

 columellar and parietal margins are heavily calloused ; columella rather 

 wide, concave and flattened. 



Length 4.7, diam. 4.6 mm. 

 " 4-3 " 4 



Mouth of the Arroyo San Juan, where it empties into the La Plata, 

 Province of Colonia, Uruguay (type locality); San Gabriel's Island in the 

 La Plata, near Colonia, in the same Province, and Fray Bentos on the 

 Uruguay River (Wm. H. Rush). 



Development. At the end of the second whorl the periphery begins to 

 be weakly angular, the shell being about 2 mm. in diameter. Before that 

 stage the shape is globose-depressed, with a rounded periphery. At the 

 end of the third whorl the angle is strong. The columella is very broad, 



