584 PATAGONIAN EXPEDITIONS: ZOOLOGY. 



No immature stages are represented in the series before me. The 

 species belongs to the more primitive or youthful group, by its simplicity 

 of form, the absence of varices, and the want of contraction at the mouth. 

 It is not closely related to any of the La Plata drainage forms, but has 

 some resemblance to P. lapidum, from which it differs by the obtusely 

 biangular shape, the much larger aperture, and the heavily calloused 

 columella, which allies it to species of the Uruguay system, and those 

 following. 



POTAMOLITHUS INTRACALLOSUS Sp. nOV. 

 (Plate XLW, Figs. 9, $a.) 



The shell is imperforate, solid, globose with short, conic spire, olive- 

 green, blackish in the axial region. Surface very lightly marked with 

 growth lines. Whorls 43^ , convex, the last strongly convex below the 

 suture and at the periphery, which is below the middle, somewhat flattened 

 above the periphery ; base convex ; an acute keel divides off a crescentic 

 umbilical area. The suture descends slightly near the aperture. The 

 aperture is ample, oblique, very shortly oval. The peristome is black- 

 edged, not expanded. The continuous columellar and parietal margins 

 are very heavily calloused, the columella broad and flattened. In oblique 

 view in the aperture, a broadly rounded prominence is seen in the middle 

 of the columella. 



Length 3.7, diam. 3.5 mm., length of aperture 2.7 mm. 



Hiririea, Rio Ribeira, State of Sao Paulo, Brazil. Type, No. 103,047, 

 A. N. S. P., from No. 2037 f tne Museu Paulista. H. von Ihering. 



This small species of the P. buschii group is notable for its very broad 

 columella, with an internal node or rounded prominence. This last 

 feature differentiates it from P. ribeirensis, which closely resembles intra- 

 callosus in shape. 



POTAMOLITHUS RIBEIRENSIS sp. nov. 



(Plate XLI, Figs. 6, 7.) 



The shell is imperforate, rather solid, globose, red-brown, with a faint 

 olivaceous tint near the outer lip ; sculpture of indistinct growth-lines 

 only. Spire short, conic. Whorls y/ 2 , convex, the last globose, most 

 convex at the periphery and just below the suture, the intervening surface 

 somewhat flattened in some examples, but strongly convex in others. 

 The periphery is very indistinctly subangular in front, but in fully adult 



