44 TERRESTRIAL MOLLUSCA INHABITING SOCIETY ISLANDS. 
species; and nowhere else do we find so great a variety of forms in the shape of the 
shells, which varies from the almost globose P. Hebe to the slender P. elongata. It is 
no less noteworthy that nearly one-half of the species have a “ button-like ” tooth on 
the parietal wall, and some have a tooth-like projection on the inner margin of the 
peristome, which gives the aperture an auriculate shape—a feature found in no other 
group. The Society Islands shells are also the most variable in color, and more than 
half of the species are more or less spirally banded—a character rarely found in the 
extra-limital species. 
The distribution of the various species throughout the group presents many very 
interesting features, which are, indeed, worthy of more attention than I am able to 
give to the subject. With three exceptions, each island is inhabited by distinet species, 
and some possess peculiar types or subgenera. The specific centre or metropolis of 
nearly all the species is clearly defined by the profusion or concentration of indi- 
viduals in limited areas. In some instances we find two, rarely three, species having 
their centres of distribution in a single valley, and in some cases one is entirely 
restricted to its headquarters, whilst the others have spread into two or more valleys. 
On Tahiti, the largest island in the group, we find eight speces only, six of which 
are endemic. One (P. clara), which has a limited range, appears to be gradually 
becoming extinct. Four species (P. filosa, nodosa, producta and stolida) are each 
restricted to a single valley. All the above species are well-defined, and exhibit but 
little variation. On the contrary, P. Otaheitana, which has its centre of distribution 
in Fautana valley, has spread all round the island, and is subject to so much variation 
that no less than fourteen species have been proposed for the different forms. ‘Two 
species (P. hyalina and attenuata) are common to other islands ; the former, as before 
mentioned, is found in the Austral and on one of the Cook’s group ; the latter occurs 
on Raiatea, but does not inhabit the two intermediate islands. It is a noteworthy 
fact that, notwithstanding both species have spread nearly all round Tahiti, yet they 
have not developed a single varietal feature, but, on the contrary, are remarkably 
uniform in all their specific characters. One would naturally suppose that the southern 
examples of P. hyalina, which are subject to lower temperature and different forma- 
tion (elevated coral reefs), would have exhibited some degree of variation to distinguish 
them from the Tahitian specimens living in a higher temperature on a volcanic 
island. 
Here we have three species ranging round the island, and all subject to the same 
conditions of life, yet two have not shown the slightest tendency to depart from the 
typical forms, and the other, which is very variable in its metropolis, has developed 
many local varieties. The above facts, which are common to other species, seem to 
suggest that physical conditions are not the primary cause of variation, but that it 
is the operation of some unknown law. 
Moorea, which is separated from Tahiti by a channel only eight miles wide, is 
