PILSBRY: NON-MARINE MOLLUSCA OF PATAGONIA. 597 



POTAMOLITHUS HIDALGOI Pilsbry. 

 (Plate XXXIX, Figs. I, la, i.) 



Potamolithus hidalgoi Pilsbry, Nautilus, X, p. 87, Dec., 1896. 



The shell is imperforate, trochoidal, with flattened base and conic spire ; 

 moderately solid. The type specimen is dull reddish-brown, somewhat 

 olive-tinted at the base, and olivaceous blackish on the lip-varix ; . but 

 some immature shells of the original lot are dull olive-colored, darker on 

 the keel. The surface is slightly marked with growth-lines. The spire is 

 conic. There are 4^ whorls, the first 2^ convex; then a keel appears, 

 immediately above and filling the suture. The last two-thirds of the last 

 whorl descends slowly, the peripheral keel projecting above the suture. 

 The last whorl is flat and slopes steeply above the peripheral keel, and is 

 slightly convex below it. A very narrowly crescentic columellar area is 

 defined by an inconspicuous, but acute angle. The outer lip is strength- 

 ened by a moderately strong varix, not continued below the termination 

 of the peripheral keel. The aperture is extremely oblique, rounded-ovate, 

 angular above and indistinctly so outwardly, at the termination of the 

 keel. The columella is arcuate and moderately calloused. 



Length 5.2, diam. 5 mm. 



Uruguay River, at Paysandu, Uruguay. Types, No. 69,687, A. N. S. P. 



A very weak ridge below and near the suture may be seen in two 

 immature shells of the type-lot, but this is not developed in the others. 

 The early neanic substage is Naticoid, like the corresponding age in P. 

 microthaiima. The rest of the neanic stage is the equivalent of substage 

 2 in P. microthaiima. The deep descent of the last whorl gives a markedly 

 gerontic character to the adult stage. P. hidalgoi is therefore less evolved 

 sculpturally in wanting the third neanic substage, senile characteristics 

 supervening earlier. 



The species is named in honor of Dr. J. G. Hidalgo of Madrid, author 

 of a beautiful work on the mollusks of the Spanish Commission to South 

 America, among many other important labors. 



POTAMOLITHUS MICROTHAUMA Pilsbry. 



(Plate XXXVIII, Figs. 2, 20, 26, 3.) 

 Potamolithus microthauma Pilsbry, Nautilus, X, p. 87, Dec., 1896. 



The shell is imperforate, biconic, very solid and strong. The last whorl 

 is olive-green, rather profusely marked with irregular buff maculae, which 



