aoe INTRODUCTION. 
West Indies and their neighbourhood; for Achatinella in the Sand- 
wich Islands; for Partula in the Pacific Islands, south of the equator; 
to the United States of America we look for Helices with toothed 
apertures; to the Philippine Islands for the ivory and beautifully 
painted species, &c., and we venture to call them stragglers, if they 
are brought to us from any other quarter. 
Dr. Pickering remarks, in relation to the Feejee Islands, “It was only 
here, in the midst of the Coral sea, where I found myself surrounded 
by a great variety of Cone, Mitre, Olive, Cowry, Ovula, Harpa, 
Terebra, Cassis, Strombus, Coneelix, Pyramidella, Tridacne, Vulsella, 
Lima, &c., that I became fully aware of the imperfect state of this 
science. We missed Patella, Eburna, Terebellum, Cancellaria, Hip- 
popus, Ancillaria, and Marginella. Bivalves seem to prevail less than 
at Tonga. Mactra proper was not met with. In fluviatile shells 
these islands are richer than the eastern ones, no doubt on account of 
their larger size and the consequent greater abundance of fresh water. 
A fresh-water bivalve, Cyrena, was here for the first time met with 
among the islands. Among land-shells we missed Partula. The 
appearance of large Bulimi reminded one of the continent.” The true 
Helices seem to be supplanted by Nanina. 
Another point of interest, extensively elucidated by the collections 
of the Expedition, is the occurrence of analogous species in co-ordinate 
regions. It is now a received fact that the animals and plants of the 
northernmost zones are, for the most part, identical throughout the 
whole circuit; and that the species gradually diverge from each other 
towards the equator, on the three continents; and that after passing 
the equator towards the south, there is not a return to the same 
species, and rarely to the same genera, as we should expect if variation 
of forms depended mainly on difference of temperature. There is, 
however, a return to mollusks of a kindred character and form, and 
oftentimes to the same genera. 
The analogies of specimens from distant regions are much stronger 
when reckoned by isothermal longitude than by isothermal latitude. 
In the latter case we may have analogous genera. Along our northern 
