578 PATAGONIAN EXPEDITIONS I ZOOLOGY. 



POTAMOLITHUS GRACILIS VIRIDIS Pilsbry. 

 (Plate XLI, Figs. 9, ga.) 



Potamolithus gracilis mridis Pilsbry, Nautilus, X, p. 89, Dec., 1896. 



This form is similar to gracilis in contour, etc., but there is a deeper 

 sinus near the upper termination of the lip, and the basal sinus is well 

 developed. The color is green. The dark maculae shown in the figure 

 are due to the dried soft parts. 



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



POTAMOLITHUS SIMPLEX sp. nov. 



(Plate XXXIX, Figs. 6, 6a.) 



The shell is acutely ovate-conic, thin, but rather solid, pale greenish- 

 yellow ;.the surface glossy, faintly marked with growth-lines. The spire 

 is straightly conic, apex rather acute. Whorls 5^ , convex, the last evenly 

 globose, not expanded at the lip. There is a distinct and rather wide flat 

 axial area, bounded by a delicate keel. The suture is deep and descends 

 briefly and rather abruptly close to the aperture. The aperture is oblique 

 and acutely ovate. Peristome simple, thin, black-edged and continuous, 

 its edge even, not in the least sinuous or notched. The columella is very 

 narrow and concave. 



Length 4.3, diam. 3, length of aperture 2.25 mm. 



Uruguay River at Paysandu. 



The slender ovate contour and smooth surface ally this species to P. 

 gracilis, but the total absence of any sinus or notch in the lip, the anterior 

 descent of the suture and the well developed axial or umbilical area are 

 features unlike P. gracilis. No young individuals have been identified, 

 but they probably could not be distinguished from P. gracilis. P. simplex 

 approaches Littoridina in contour, but the anteriorly descending suture 

 and the axial crescentic area are unlike that genus. 



GROUP OF P. BUSCHII. 

 POTAMOLITHUS AGAPETUS sp. nov. 



(Plate XL, Figs. 10, ioa.) 



The shell is imperforate, globular-conic, of a rather light green color. 

 The surface is smoothish, faintly marked with growth-lines. Spire short 



