PILSBRY: NON-MARINE MOLLUSCA OF PATAGONIA. 607 



In the fresh specimens from 65 miles north of the Rio Chico the right 

 valve has below the beak a horizontal arcuate tooth, with a straight 

 oblique tooth above it nearer the anterior (short) end. The laterals appear 

 to be smooth. The left valve has a rather long slender tooth, lower and 

 angulated in the middle, the anterior ramus stouter and longer than the 

 posterior. The lateral teeth arise under the visible part of the umbones 

 (while in P. patagonicum they arise beyond it). They are high and tri- 

 angular in profile (fig. 16). The shells are light olive externally, with 



FIG. 1 8. 



P. magellanicum, 65 miles above Rio Chico. Interior of right and left valves. 



several darker concentric streaks. The largest shells measure, length 4.9, 

 alt. 4, diam. 2.7 mm. 



Specimen from a spring 25 miles above the Sierra Oveja are similar, 

 except that the lower cardinal tooth of the right valve is stouter. One 

 opened contained eleven young ones about 1.6 mm. long, almost filling 

 the cavity. 



The teeth are practically identical in specimens from six lots examined. 



PlSIDIUM PATAGONICUM Sp. nOV. 

 (Plate XLVII, Figs. 8-10.) 



The shell is pale buff, glossy, very finely striate, with low, wide, smooth 

 and glossy beaks ; strongly inequilateral, the anterior end very short and 

 rounded, base evenly convex, posterior end narrow and somewhat pro- 

 duced. Interior white. Cardinal teeth are excessively weak and low, 

 nearly effaced. There is a very low, horizontal, rudimentary tooth in the 



