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 daedalea, 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 Achatinellae 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 lamellae 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. 



