608 PATAGONIAN EXPEDITIONS: ZOOLOGY. 



right valve, a low short one in the left, with the scarcely discernible trace 

 of another anterior to it. These teeth, especially the anterior one, are too 

 distinctly defined in fig. 10. Lateral teeth very short and moderately 

 strong, distant from the beaks. 



Length 4.9, alt. 4.3, diam. 3 mm. 



Springs on the Rio Chico, fifteen miles (type loc.) and thirty miles above 

 the Sierra Oveja ; twenty-five miles below the Rio Belgrano ; and in the 

 Arroyo Eke, near the head waters of Spring Creek, 2400 ft. elevation. 



This clam differs from P. magellanicum by the shorter anterior end, short 

 lateral teeth remote from the cardinals, the anterior laterals standing at an 

 angle approaching 100 with the posterior laterals, on account of the curva- 

 ture of the anterior margin. In P. magellanicum the angle of divergence 

 of the teeth is decidedly greater, and they are longer. The cardinal teeth 

 are almost obsolete in P. patagonicum, not projecting above the level of 

 the hinge. 



In the type lot, as well as in all the specimens from springs recorded 

 above, the surface is straw-yellow and the striation fine and even, without 

 periodic lines indicating growth-arrest. In specimens from small streams 

 in the same region the shell attains a greater size, up to length 6.9, alt. 

 5.8 mm., and is marked externally with several darker concentric streaks, 

 indicating periods of growth-arrest ; the color is generally paler. This 

 form, which may be called var. zonifer, is figured on Plate XLVItf, fig. 9. 

 It is from small streams on the Rio Chico fifteen and twenty-five miles 

 above the Sierra Oveja. 



PlSIDIUM OBSERVATIONIS Sp. nOV. 

 (Text fig. 19.) 



The shell is inequilateral, the beaks low and wide, anterior end very 

 short, broadly rounded, posterior end narrow, rounded. Surface glossy, 

 olive, drab or yellowish, marked with several impressed and darker rest- 

 ing periods. Interior bluish-white. Cardinal teeth : in the right valve 

 there are two narrow teeth, parallel, oblique and contiguous. In the left 

 valve there is one nearly straight horizontal tooth, lower and thinner near 

 the middle. The lateral teeth are rather long and not remote from the 

 beaks, single in the right, double in the left valve. The interlocking sur- 

 faces of these teeth are more or less granulous. 



Length 4.8, alt. 4, diam. 3 mm.; sometimes larger, length 5.1 mm. 



