586 PATAGONIAN EXPEDITIONS! ZOOLOGY. 



perforation and a feebly differentiated axial or columellar area, linear in 

 the adult stage, but becoming wider in old shells. 



Length 5.7, diam. 4.3, length of aperture 3.8 mm. 



Colony of Hammonia, State of Santa Catharina, Brazil. Types, No. 

 103,048, A. N. S. Phila., from No. 163 of the Museu Paulista. 



This is a much more robust species than P. simplex. It differs from 

 P. lapidum by the produced spire and heavier columella. It is unlike all 



(Fig. 1 6.) 



4- 



FIG. 1 6. Potamolithus catharince. 



described forms in having a distinct umbilical perforation. The apex is 

 perfect in all of the shells received, although the oldest of them has the 

 last whorl deeply eroded. 



GROUP OF P. LAPIDUM. 



POTAMOLITHUS LAPIDUM (d'Orbigny). 



Paludina lapidum d'Orbigny, Magazin de Zoologie, p. 29 (1835). 

 Paludestrina lapidum d'Orbigny, Voyage dans 1'Amer. Merid., Mol- 



lusques, p. 382, pi. 47, f. 4-9. 

 Hydrobia lapidum Strobel, Materiali per una Malacostatica di terra e di 



acqua dolce dell' Argentinia Meridionale, 1874, p. 59, with var. 



dunkeri, t. c., p. 59, pi. 2, f. 5. 

 ? Hydrobia lapidum d'Orb., E. von Martens, Malakozoologische Blatter, 



XV, 1868, p. 192 (Guahyba River at Porto Alegre ; near Roders- 



berg; in the forest region and Cima da Serra at the Estancia of 



Christian Horn, on the plateau, 3-4000 ft. elevation; collected by 



Dr. Hensel). 

 ? Lithoglyphus lapidum d'Orb., von Ihering, Malakozoologische Blatter 



(neue Folge), VII, 1885, pp. 96-99, figs. 1-3 (dentition, head and 



