606 PATAGONIAN EXPEDITIONS: ZOOLOGY. 



The shell is fragile, olive-gray when full-grown, the young ones grayish, 

 with a yellow zone at the basal edge. The ends are more abrupt than 

 in M. patagonicum, and the beaks fuller. The teeth are decidedly more 

 delicate and compressed than in M. patagonicum. 



PI. XLVIa, fig. 7, *ja. Length 9, alt. 7.8, diam. 5 mm. 



" XLVIa, " 6. " 5.3 " 4.2 " 2.9 " (immature). 



Strobel reports this species from San Carlos, Province of Mendoza, 

 Bahia Blanca and Carmen de los Patagones. He gives the measurements, 

 length 9, alt. 7.5, diam. 6 mm., for an example from the last named locality. 



From the Rio Camaguan, Rio Grande do Sul, Dr. von Ihering sent a 

 single specimen similar to M. argentinum, except in being shorter and 

 more globose; length 7.9, alt. 7, diam. 5.2 mm. If such proportions 

 characterize a race in that river, it will probably be considered as speci- 

 fically distinct. 



PlSIDIUM MAGELLANICUM (Ball). 

 (Plate XLVII, Figs. 12-16.) 



Corneocyclas magellanica Dall, Bulletin of the Museum of Comparative 

 Zoology, Vol. 43, October, 1908, p. 411. 



"Shell small, whitish, with an olivaceous smooth periostracum, low, 

 wide beaks and polished surface, with faint concentric indications of three 

 or four resting stages ; form inequilateral, anterior end shorter, bluntly 

 subtruncate ; base evenly rounded ; posterior end slightly attenuated and 

 rounded; external sculpture of faint incremental lines, chiefly obsolete 

 between resting stages ; interior smooth, white ; hinge of right valve with 

 a single feeble horizontal tooth directly under the beak, and two well- 

 developed lateral teeth rather distant from the beak, the posterior lateral 

 stronger. Length of shell 3.5, of posterior end of shell 1.8 ; height 2.5 ; 

 diameter (of both valves) 2 mm." (Dall). 



Magellan Straits in 61 fathoms, "Albatross" Station 2778. "A single 

 right valve, evidently washed into the sea from some stream" (Dall). 



Springs on the Rio Chico de Santa Cruz, fifteen (PI. XLVII, figs. 15, 

 1 6) and twenty-five miles above the Sierra Oveja ; Rio Blanco, at base of 

 the Andes ; springs near base of the Andes, 65 miles north of the Rio 

 Chico, 2400 ft. elevation (PI. XLVII, figs. 12-14). 



Dr. Dall, who kindly compared specimens from the last locality men- 

 tioned above with the type of P. magellanicum, states that they agree 

 almost exactly and, in his judgment, are the same species. 



