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Show 1871.] MR. HARPER PEASE ON POLYNESIAN LAND-SHELLS. 451 As to general variation I note one fact. The species of most of the genera inhabiting Southern and Western Polynesia, ranging over a distance of more than 5000 miles from the Pelews to the Marquesas, vary less from a common type than those on the Hawaiian Islands, which are restricted in their distribution to 300 miles. Partula, the prevailing genus of Southern and Western Polynesia, occurring on all the high islands, presents so little variation that not a single subgenus has been proposed ; while at the Hawaiian Islands the genus Helicter varies more on any one of the islands, even the smallest, but eight miles in length, than the Partulee throughout their whole range. I select the two genera above in illustration, as the species are the largest in size, and comprise together nearly one half of all the land-shells inhabiting Polynesia. I now offer a few remarks on the genera, adding to each descriptions of such species as I find in m y collection to be new. Genus PITYS. Pitys, Beck, Index Molluscorum, 1837, p. 9. The above genus was established by Dr. Beck on Helix oparica, Anton, from the collection made by the late Mr. Cuming at the island of Rapa (Opara), one of the Austral group, a few hundred miles south of Tahiti. There is no doubt as to the identity of the species, although it was described by Dr. Anton as H. oparica, from America. By reason of the similarity between the shells of certain species discovered since and those of the European genus Discus, Fitz.= Patula, Held., all the Polynesian forms have been described under the European genus; with few exceptions the shells are quite distinct, and the animal decidedly so; they are most numerous at the Hawaiian and Tahitian Islands, less so at the Samoas, and altogether absent in West Polynesia. The species are quite uniform throughout their whole range. The following are their general characters :- " Shell orbicular or planorboid, finely radiately ribbed ; spire but slightly elevated; last whorl rounded at its periphery and also at the umbilicus, more or less openly umbilicate, rarely imperforate; aperture generally dentate or laminate ; radiately striped or tessellated on their upper surface with reddish brown and yellowish, the stripes occasionally taking a zigzag form on the periphery and base; rarely wholly reddish brown; generally covered with a thin epidermis, which, on a few species, supports short hairs." Only three species, so far as known, are imperforate, and those the smallest of the genus, viz. imperforata, Pse., rotula, Jacq., and oparica, Anton; of the 37 species in the following catalogue, the aperture of 29 is dentate or laminate. The following is the only variation from the type as above :-H. stellula, Gld., inhabiting the Hawaiian Islands, is depressed, carinate at the periphery, and strongly ribbed, and of a shining texture. At the Tahitian Islands a group of species of an aberrant form |