MOLLUSCA. 193 



of late been afforded by the microscopic investigation of their 

 structure by Dr. Carpenter, an investigation which is still in 

 progress. That gentleman observes, "that marked differences 

 in the structure of shell go along with marked difference in 

 general characters, and that a close correspondence in the 

 structure of the shell may be held to indicate a tolerably close 

 natural affinity."* And he enumerates certain genera "which 

 may be at once distinguished from each other, and from all 

 other shells, by the characters supplied by a fragment of shell 

 which a pin's head would cover." Should more extended 

 observations warrant the broad inferences to which such in- 

 quiries at present point, and be found applicable to the Crus- 

 tacea and Echinodermata, no less than to the Testacea, how 

 clear is the light which they will cast into "the palpable 

 obscure," which sometimes baffles the most anxious and 

 persevering efforts of the geologist I 



Another series of observations, of a nature totally unlike 

 these, has given additional importance to the shells of stratified 

 rocks, by teaching us better to understand the circumstances 

 under which they have been originally deposited.- These 

 investigations were carried on by Professor Edward, Forbes, J* 

 in the ^Egean Sea, on board H. M. S. Beacon^ Captain Graves, 

 and continued for eighteen months. By means of the dredge, 

 the MoUusca and Kadiata of that region were explored, at all 

 depths of water between the surface and 230 fathoms. Nearly 

 700 species were thus found, and, in different regions of depth, 

 they were associated in such a manner that each of these 

 regions presented its own peculiar and characteristic association 

 of species, just as on lofty mountains the character of the 

 vegetation changes in proportion to the altitude. Those 

 species which had the widest range of geographical distribu- 

 tion, had also the most extensive range with regard to regions 

 of depth ; and some were discovered living, which had pre- 

 viously been known only as fossil. Both with regard to 

 vegetable and animal life, species were found to attain, at 

 certain depths, a maximum size, then gradually to diminish, 

 and finally to disappear, their places being supplied by similar 

 forms, specifically distinct. Genera, in like manner, were 

 found to be replaced by corresponding genera. So that the 



* Annals Nat. Hist. December, 1843. 



f Report to British Association. Cork meeting, 1843. 



