Remarks on Mollusca and Shells. 77 



for Cylindrella in the West Indies and their neighbourhood ; 

 for Achatinella in the Sandwich 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 we 

 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, Coivry, Ovnla, Harpa, Terebra, Cassis, Strombus, 

 Conoelix, Pyramidella, Tridacna, Vulsella, Lima, &c, that 

 I became fully aware of the imperfect state of this science. 

 We missed Patella, Eburna, Terebellum, Cancellaria, Hip- 

 popus, Aneillaria, and Marginella. Bivalves seem to pre- 

 vail 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 conse- 

 quent greater abundance of fresh water. A fresh- water 

 bivalve, Cyrena, was here met with for the first time 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. 



4, Analogous Species in co-ordinate Regions. 



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 north, 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 tem- 

 perature. There is, however, a return to molluscs of a kin- 

 dred 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 



