PILSBRY: non-marine MOLLUSCA of PATAGONIA. 605 



MUSCULIUM PATAGONICUM Sp. nOV. 

 (Plates XLYla, Fig. 8 ; XLVII, Figs. 1-7.) 



The shell is thin, nearly equilateral, pale straw-yellow, glossy, finely 

 striate. The beaks are low and broad, the embryonic stage marked off by 

 a contraction or gutter. Anterior end almost symmetrically rounded; pos- 

 terior end slightly flattened or subtruncate. Hinge-line arched. Cardinal 

 teeth very minute, double in the right, single in the left valve. Lateral 

 teeth very short, triangular, single in the left, double in the right valve. 



Figs. 6, 6a. Length 8, alt. 6.6, diam. 4.3 mm. 35 miles above Sierra Oveja. 

 " 4,5 " 8.8 " 6.8 " 4.5 " 25 " 



II Q II h^ " [^ " CCi " " " " 



Springs and small streams along the Rio Chico de la Santa Cruz, from 

 15 to 50 miles above the Sierra Oveja. Types from 50 miles above the 

 Sierra Oveja (PI. XLVI^-, fig. 8). 



This species is closely related to Muscidutm argentiuum (d'Orbigny), 

 but constantly differs from that by having the posterior end less abruptly 

 truncated, and the beaks flatter, not so full. Figs. 2, 3, 5 are not very 

 good. Figs. 6, da and 7 show the shape better. Fig. i well shows the 

 teeth as seen in a partly open shell. Eight lots are before me, from as 

 many springs and streams, at distances of 15, 25, 30, 35 and 50 miles 

 above the Sierra Oveja. The examples from farther up are the largest 

 and are remarkably well developed in every way (PI. XLVIrt-, fig. 8). 



MuscuLiUM ARGENTiNUM (d'Orbigny). 

 (Plate XLVIa, Figs. 6, 7, -ja) 



Cyclas aygeutina d'Orbigny, Mag. de Zool., 1835, p. 44 ; Voy. dans I'Amer. 



Merid., Mollusques, p. 568, pi. 83, figs. 5-7 (Montevideo). 

 Sphceyhini aygentinmn d'Orb., Strobel, Mat. Malacostat. Argent., p. 77. 



Not Pisidiuni argeutimmi Clessin, Conchyl. Cab., p. 63, fig. 7.a. 

 D'Orbigny's figures of this species are very unsatisfactory. Clessin has, 

 I believe, entirely misunderstood them. His Pisidiuni argcutiuitni may pos- 

 sibly be P. steykiaiiuni Pils. For the purpose of affording a basis for com- 

 parison with Argentine and Patagonian species, I figure two topotypes, an 

 adult and a half-grown shell, collected by Dr. \V. H. Rush, U.S.N., from 

 a creek in the Prado, Montevideo. 



