588 PATAGONIAN EXPEDITIONS: ZOOLOGY. 



P. lapidum (Plate XLI, fig. 5). The shell is solid, globose, with conic 

 spire longer than in P. lapidum, though the apex is eroded in all speci- 

 mens seen. Aperture smaller than in lapidum, somewhat contracted by 

 an internal thickening and contraction of the lip above. Columella 

 narrow. A very narrow umbilical area is defined by an angle, which runs 

 very close to the columellar lip. This race may be called var. elatior. 



Length 4.5, diam. 3.5 mm. 



The long spire separates this form at once from P.paysanduanus, which 

 moreover differs in the shape of the last whorl. In old examples the 

 aperture is more contracted than in that figured, and the spire is more worn. 



POTAMOLITHUS LAPIDUM SUPERSULCATUS Pilsbry. 

 (Plates XXXIX, Figs. 7, ja ; XLI, Fig. 7.) 



Potamolithus lapiditm stipersulcatiis, Pilsbry, Nautilus, X, p. 88, Dec., 1896. 

 The shell is like P. lapidum in its globular shape. It is dark green, 

 usually with pale, irregular zigzag markings, sometimes confined to the 

 spire, and there is usually an indistinct brownish band above the pe- 

 riphery. On the last whorl an obtuse ridge revolves not far below the 

 suture, most prominent on the back ; this is followed by a concavity, 

 below which there may be a second obtuse angle. The base is quite full 

 and rounded. The suture descends suddenly to the aperture, which is 

 therefore a little contracted, oblique and ovate. The peristome is acute, 

 dark-edged, continuous, without trace of a varix. The columella is 

 narrow and concave, and the parietal wall is only moderately thickened. 

 Whorls 4^ . 



Length 5.3, diam. 4.7 mm. 



" 4-3 " 4 " 



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

 Also Uruguay River at Fray Bentos, and Paysandu, Uruguay. 



Development. The spiral ridges appear only on the last whorl, and 

 usually only on its last half. Up to that time the whorls are rounded and 

 the shape Naticoid. At all stages of growth the columella is quite narrow. 



Potamolithus I. supersulcatus is quite closely related to P. tricostatus 

 (Brot), from which it differs chiefly by the weaker spiral ridges, which are 

 only one or two in number, not three, as in P. tricostatus. These ridges 

 are not only less emphatic in supersukatus^ but they do not appear so 



