Potamopyrgus spelæus, Frauenfeld, sp.

Hydrobia spelæa, Frauenfeld, Verh. Zool. Bot. Gesell. Wien, vol. xiii. (1862), p. 1022; vol. xv., p. 526, pl. viii. Hydrobia spelæa, v. Martens, Crit. List. N.Z. Moll. (1873), p. 15. Bythinella spelæa, Hutton, Manual N.Z. Moll. (1880), p. 82.

This species was found together with P. reevei in moa-bone caves. A translation of the diagnosis is given by Hutton, but the dimensions are not quite those of Frauenfeld; the type is 3 mm. long by 1.6 mm. in breadth. This minute species is nearly allied to pupoides, Hutton, which, however, is mostly, but not always, smaller, and constantly of pupoid form.

The specimens in my collection I take to be almost typical were collected by Mr. A. Hamilton, now Director of the Colonial Museum, in the salt springs at Te Mahia, Hawke's Bay, and they show the same dimensions as the type specimen. A rather large form comes from the tidal part of the Wanganui River, and its dimensions vary from 3 × 1 ¾ mm. to 3½ × 2 mm. Very small specimens occur at Nelson, the largest measuring 2½ × 1¼ mm., the smallest 2 × 1 mm. The latter is the usual size of P. pupoides, but the Nelson examples are distinguished by much more convex whorls, deeper suture in consequence, and a more conoidal spire. Forms with ventricose body-whorl were collected by Mr. Chadwick on the upper Wanganui River, and in a cold mineral pool at Rotorua by Lady Frances Brown, a most enthusiastic collector. What I consider to be the same species are examples collected by Mr. Charles Cooper, of Auckland, in hot-spring water at Te Aroha. These specimens are very variable in shape, some approaching the type and measuring 3 × 1½ mm., others with a more inflated body-whorl show¼ mm. greater breadth; they all have only four whorls. This is the only instance known to me of a mollusc living in hot water in New Zealand, while it is well known that Neritina and Bithynia have been found in France living in water from 68° to 122° Fahr.

Type in the K.K. Hofmuseum, Vienna.