Suter. — Species of the Genus Potamopyrgus. 265 



examination and comparison have now convinced me that it 

 is really a very large form of P. badia. 



Considering the great variability of Potamopyrgus I refrain 

 from establishing any new species or subspecies unless for very 

 good reasons, and merely mention the localities where forms 

 differing considerably from the type have been found. Tem- 

 perature, chemical composition, movement and size of the 

 water-area, and food available have a great influence on the 

 growth of fresh-water shells, and it is difficult to find the same 

 form of a species in more than two or three localities. Taking 

 into consideration the polymorphism of some species, and the 

 great variability, we can congratulate ourselves on having not 

 more synonyms to record. 



Type in the U.S. Nat. Museum, Washington. 



Potamopyrgus egenus, Gould, sp. 



Amnicola egena, Gould, Proc. Bost. Soc. Nat. Hist., vol. iii. 

 (1848), p. 75. Amnicola gracilis, Gould, U.S. Expl. Exp., 

 vol. xii. (1852), p. 127, figs. 151 a, b. Hydrobia egena, v. 

 Martens, Crit. List. N.Z. Moll. (1873), p. 15. Bythinella 

 egena, Hutton, Manual N.Z. Moll. (1880), p. 82. 



I am indebted to Dr. W. H. Dall for the following informa- 

 tion (in lit.) : ' The shell first described by Gould as Amnicola 

 egena is imperfect. It was inadvertently called gracilis in the 

 final report. It appears to be identical with specimens called 

 gracilis in the collection, except that the type is brown and 

 the others greenish." Gould's diagnosis is copied in Hutton's 

 Manual, and the type was found on Banks Peninsula. 



This is undoubtedly a good species, but it seems to be rather 

 rare. I have it in my collection from three localities only : 

 Two specimens from Kaiwarra Eiver, near Wellington ; one 

 from Nelson ; and fourteen from Little Kiver, Banks Peninsula. 

 The graceful elongated shape, the convex whorls, and the last 

 whorl amounting to half the length of the shell, distinguish it 

 at once from the other species. No carinated or spinous forms 

 are known. The dimensions given by Gould are : Length, 5 mm.; 

 breadth, 2J mm. ; with five whorls. The dimensions of the Little 

 River specimens, with five whorls, are 4 x 2 mm. ; those of the 

 Kaiwarra specimens, 4 x If mm. with six whorls, and 5 x 2£ mm. 

 with seven whorls ; while the Nelson specimen measures 5| x 

 2 J mm., and has six whorls. Thus my specimens must have 

 slightly shorter whorls and a somewhat narrower spire than 

 the type. In all specimens the last whorl is a trifle longer than 

 half the axis of the shell. 



Type in the U.S. Nat. Museum, Washington. 



