INTRODUCTION. xv 
or less mingled. It must also be considered, that the Navicella, and 
more especially Neritina, is oftentimes decidedly littoral, and even 
marine, in its habits. 
The little island of Metia, or Aurora Island, to the northeastward of 
Taheiti, is one of peculiar interest. It is a coral island, which has been 
elevated 250 feet, or more, and has no other high island anywhere 
near it. On it were found four small land-shells belonging to three 
genera, viz. :—Helix pertenuis, Helix deedalea, Partula pusilla, and 
Helicina trochlea. None of these were found upon any other island. 
They seem to have originated there, after the elevation of the island, 
and have a significant bearing upon the question of local and periodical 
creations in comparatively modern times. 
As the genus Partula is characteristic of the groups just south of 
the equator, so Achatinella is the characteristic shell of the Sandwich 
Islands. Closely connected as the islands of this group are, they each 
have their peculiar forms of land-shells ; and, as the southern islands 
bear evidence of greater age than the northern ones, we may infer 
that, within these narrow limits, we have evidence of the appearance 
of some species subsequent to the existence of others now living. On 
the Island of Kauai, the oldest of the group, we have Achatina adusta 
and pyramidata, a form which does not appear on the other islands ; 
the Achatinellze are chiefly of the elongated glabrous form, which I 
have grouped under the name Leptachatina; the Helices are planor- 
boid and multispiral. On Molokai, the species of Achatinella are 
large and beautiful, and peculiar in their form and colouring. On 
Maui, the Helices are small and glabrous, with some very curious 
hispid and ribbed species, with lamella within the aperture. On Oahu, 
the species of both Helix and Achatinella are similar to those on 
Maui. On Hawaii, Succinea seems to prevail in larger proportion 
than on the other islands, while Achatinella, which occurs so abun- 
dantly on all the other islands, either does not occur at all, or but 
very rarely. 
Boston, December, 1851. 
