Potamopyrgus antipodum, Gray, sp. (em.).

Amnicola antipodanum, Gray, in Dieffenbach's “New Zealand,” vol. ii. (1843), p. 241. Hydrobia antipodum, v. Martens, Crit. List N.Z. Moll. (1873), p. 14. Hydrobia antipodum, E. A. Smith, Voy. “Erebus” and “Terror,” Zool., vol. ii. (1875), p. 3, pl. i., fig. 19, above. Bythinella antipoda, Hutton, Manual N.Z. Moll. (1880), p. 81. Potamopyrgus antipodum, Hutton, Trans. N.Z. Inst., vol. xiv. (1882), p. 145, pl. i., figs. C, G (except upper right figure). Potamopyrgus antipodum, Hedley and Suter, P.L.S. N.S.W. (2), vol. vii. (1893), p. 619. Potamopyrgus antipodum, Suter, Journ. de Conch., vol. xli. (1893), p. 221.

The statement made by v. Martens (l.c., p. 141) that “some specimens are bristly” is not correct. This species has never been found with spines or a carina.

The diagnosis of the species may be found in Hutton's Manual, and in Trans. N.Z. Inst., vol. xiv.

P. antipodum is just as variable as most species of the genus, especially in size; but the conical form, flatly convex whorls, and little-impressed suture are fairly constant characters.

I have specimens from twelve localities, ranging from Auckland to Southland. Specimens from Owaka (Clutha) are very small and ventricose, with five whorls only, and measuring 4½ × 2¾ mm. Another dwarf form was collected by Mr. R. Murdoch, of Wanganui, on the Waimate Plains.

The great majority of the specimens are covered with a black coating. It is remarkable that P. antipodum is very often met with in brackish water, and almost without an exception such specimens have the tip of the shell eroded. In some places I saw this species living on Ulva plants. I have specimens from brackish water from seven different localities.

Type in the British Museum.

Potamopyrgus antipodum, Gray, subsp. zelandiœ, Gray, subsp. Amnicola (?) zelandiœ, Gray, in Dieffenbach's “New Zealand,” vol. ii. (1843), p. 241. Hydrobia zelandiœ, v. Martens, Crit. List. N.Z. Moll. (1873), p. 15. Hydrobia zelandiœ, E. A. Smith, Voy. “Erebus” and “Terror,” Zool., vol. ii. (1875), p. 3, pl. i., fig. 19, below. Bythinella zelandiœ, Hutton, Manual N.Z. Moll. (1880), p. 81.

This subspecies is fairly well represented by the upper right figure on pl. i., fig. C, in Trans. N.Z. Inst., vol. xiv. It is distinguished from the species by its somewhat smaller size and more tapering form. Dimensions of type are: Length, 5mm.; breadth, 2½ mm. It is much rarer than antipodum, but also shows a good amount of variation.