1871.] MR. HARPER PEASE ON POLYNESIAN LAND-SHELLS. 4.51 



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 Ha- 

 waiian 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 Partulce 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 descrip- 

 tions of such species as I find in my 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 di- 

 stinct, 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 



