x ii 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 theFeejee 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, Coriselix, 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 



