566 PATAGONIAN EXPEDITIONS I ZOOLOGY. 



POTAMOLITHUS Pilsbry. 



Paludestrina in part, d'Orbigny, Voyage dans 1'Amerique Meridionale, 



Mollusques, p. 381. 

 Lithoglyphus sp., of some authors. 

 Potamolithus Pils., Nautilus, X, Nov., 1896, p. 80. 



Amnicolidae with the shell imperforate, solid, ovate or globose, smooth 

 or 1-3 carinate, covered with a thick cuticle, which is usually green or 

 olive ; aperture ovate or rounded, the peristome continuous ; columella 

 concave, more or less heavily calloused. 



Operculum lodging some distance within the aperture, corneous, oval, 

 reddish-brown, opaque, with a thinner, yellowish border along the basal, 

 outer and upper margins. It is composed of about 2 whorls, the nucleus 

 near the lower third, and nearer the columellar side. The outer face is 

 slightly concave and rather strongly striate. Inner face is glossy except 

 for a long dull scar of attachment near the columellar margin (P. rushii]. 



Penis simple, terminating in a small glans surrounded by a fleshy pre- 

 putial ring. 



The radula has teeth of the form usual in AmnicolicUe^ central tooth 

 with 2 to 4 basal cusps on each side, admedian tooth armed with 8 to 10, 

 marginal teeth having many cusps, 17 to over 30. 



Type P. rushii Pilsbry. 



Distribution, La Plata drainage and faunally similar streams draining 

 into the Atlantic in Sao Paulo and Rio Grande do Sul, Brazil. 



FIG. 13. 



"Pen. 

 Head, showing tentacles and penis, and end of the penis of Potamolithus. (After von Ihering.) 



The soft anatomy is known from A. d'Orbigny's figures of living P. 

 lapidum and H. von Ihering's description and figures of a form from near 

 the mouth of the Santa Maria River, of the Rio dos Sinos drainage, identi- 



