species to three, and adding a new species (P. pupoides). Hutton says: “The absence of books prevents me feeling certain that all the synonyms I have given are correct.” With regard to books we are not much better off than we were twenty-two years ago, and, besides this, there is the very great inconvenience for us that the types of all the species, Hutton's species excluded, are in foreign museums. It was many years back, when material in my collection was fast accumulating, that I found Hutton's restriction to a total of only four species unsatisfactory. I am fully aware of the great variability of fresh-water molluscs, also of the fact that many species of Potamopyrgus are polymorphic, and therefore one and the same species may have been described under different names. There is a spinous angulate form, then an angulate espinous form, and thirdly an acuminate ecarinate one. Of the New Zealand species only two are polymorphic.

I tried to get as much information as I possibly could about those species of which I did not possess sufficient knowledge, and I have to thank especially Dr. W. H. Dall, Hon. Curator of Mollusks, U.S. National Museum, Washington; also Dr. H. Fischer, of Paris; Dr. R. Sturany and Dr. Oberwimmer, K.K. Hofmuseum, Vienna, for the great readiness with which they acceded to my request. The revision now undertaken is to a large extent based on the information thus obtained, and I hope it may prove useful to students of conchology.

The species of Potamopyrgus described from New Zealand up to now number eleven (omitting crossei, Frfld., ciliata, Gould, and gracilis, Gould, for reasons shown later on), and they were formerly classed under five genera: Melania, Amnicola, Paludestrina, Hydrobia, and Bythinella. These eleven species I now reduce to five, with three subspecies.

Genus Potamopyrgus, Stimpson (1865); Stimpson, Amer. Journ. of Conchology, vol. i. (1865), p. 53; Smithon, Miscell. Coll., No. 201 (1865), pp. 49, 50.

Type: Melania corolla, Gould, Proc. Bort. Soc. Nat. Hist., vol. ii. (1874), p. 223.

Synonym: Pyrgophorus, Ancey, Bull. Soc. Mal. France, vol. v. (1888), pp. 188, 192.

Stimpson's diagnosis of the genus is reproduced by Hutton in Trans. N.Z. Inst., vol. xiv., p. 143, with some additional remarks on the dentition.

Pilsbry says, “Potamopyrgus is a genus of great antiquity, extending at least as far back as the early Eocene. It now comprises all of the fresh-water rissoids of New Zealand, a majority of those of Australia, with species in West Africa and