596 PATAGONIAN EXPEDITIONS: ZOOLOGY. 



Uruguay River, at Paysandu, Uruguay. Types, No. 69,695, A. N. S. P. 



Development. The series at hand contains no young shells, one only 

 (PI. 38, fig. 8) has not yet formed the post-variceal contraction. It ap- 

 pears however that, until the last whorl is reached, the shell has the primi- 

 tive Naticoid shape. The last whorl represents morphologically the 

 second neanic substage of such accelerated forms as P. microthaiona. The 

 strong post-variceal contraction and heavy thickening of the inner margin 

 of the peristome declare that the last stage is distinctly gerontic. 



P. diiioc/ti/its differs conspicuously from P. microtliaiima, Jiidalgoi and 

 peristoinatns by the absence of a peripheral keel, and from P. orbiguyi 

 by the shape of the last whorl and the ill-developed columellar area. It 

 has perhaps more in common with the P. lapidinn group, especially in the 

 coloration and the persistence of the Naticoid form to the beginning of the 

 last whorl ; but the very high, massive lip-varix of P. dinocJiilus is a fea- 

 ture unlike any of the lapiihoii group. 



POTAMOLITHUS PERiSTOMATUS (d'Orbigny). 



PahuKna peristomata^ d'Orbigny, Magazin de Zoologie, p. 29, 1835. 

 Palitdcstrina pcristomata, d'Orb., Voy. dans 1'Amer. Merid., Moll., p. 382, 

 pi. 47, f. 1-3. 



"The shell is short, trochoidal, thick, smooth, carinate in front, the 

 carina projecting. Spire conic, short, obtuse at the summit, composed of 

 5 convex whorls, of which the last is keeled in front, the carina forming a 

 border above the suture between the other whorls. Aperture round, much 

 expanded, with thick, reflexed borders; the columella wide and flat. 

 Operculum corneous, spiral. Color uniform greenish, paler in front of the 

 mouth. 



"Alt. 5, diam. 5 mm." (d'Orbigny). 



Pardua River, above its confluence with the Paraguay River, at the 

 villages of Itaty and Iribucua, Province of Corrientes, Argentina, at extreme 

 low water, under stones where the current is strong; living in numerous 

 families (d'Orbigny). 



In general shape this species resembles P. bnscliii and P. conicns, but it 

 differs from both by the well-expanded peristome. It is known by the 

 original lot only. 



