580 PATAGONIAN EXPEDITIONS: ZOOLOGY. 



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

 differs by the longer, Atmiicola-X^t shape and the small aperture. There 

 is no differentiated umbilical area. 



POTAMOLITHUS BUSCHII (Ffld.). 

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



LitJioglypJnts bnschii 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>< , 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. lapidnm sitper- 

 sidcatiis. 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. 



