1897.] NATURAL SCIENCES OF PHILADELPHIA. 291 



abruptly denning regions of diverse physical features. The em- 

 bouchure of the Plate River may, as Dall has suggested, mark the 

 southern extension of typically Antillean forms, but the endemic 

 southern forms, it seems, extend both to the north and south of it. 

 The main exponent of this southern fauna is, of course, d'Orbigny, 

 whose bulky tome has been of such inestimable value to all later 

 students of South American mollusks. 



When we come to the region of Magellan Strait a good many 

 additional forms appear, and the literature is more copious. Among 

 recent papers may be mentioned Dall's report upon forms collected 

 by the " Albatross," Mabille & Rochebrune's Mollusks of the 

 Mission Scientifique du Cap Horn, Smith's " Alert " shells collected 

 by Coppinger, etc. 



Pisidium Sterkianum n. sp. Plate VI, figs. 1, 2, 3, 4. 



Shell somewhat inequilateral, ventricose, glossy, light yellowish. 

 Dorsal and ventral margins about equally arcuate ; anterior end 

 decidedly and broadly truncate ; posterior end moderately produced 

 and obliquely rounded. Beaks full but rather small, and not much 

 produced above the hinge-line. Surface very finely striated, becom- 

 ing a little more coarse near the basal margin ; interior grayish- 

 white. Right valve with two lamellar, slightly curved or sinuous, 

 parallel cardinal teeth, the laterals short, high and rather slight. 

 In left valve the laterals are lower and longer. Length 6, height 

 5, diam. 3'8 mm. 



From a creek in the " Prado," Montevideo, Uruguay. 



Many specimens were collected. One of those opened contained 

 numerous young, as is often observed in our northern Pisidia. 



P. Sterkianum is a large species, about the size of an average P. 

 Virginicum. I would identify it with Cyclas pulchella Orb. (not 

 Jenyns, = Pisidium Dorbignyi Clessin) were it not for the very 

 much smaller size (length 3 millimetres) of that form ; the young 

 P. Sterkianum of that size being much more compressed than Or- 

 bigny's figure of C. pulchella. C. pulchella was not among 

 Orbigny's South American shells acquired by the British Museum, 

 according to the official catalogue, and is not represented in the 

 Museum, as Mr. E. R. Sykes obligingly informs me. , 



It is likely that Clessin's description of "P. Argentinian" and his 

 figure la were from a specimen of this species ; but Orbigny's Cy- 

 clas Argentina, which Dr. Rush collected at the origiual locality, is 

 a, true Sphwrium, not unlike S. {Calyculina) lacustre in general ap- 



