30 PEOCEEDIKGS OF THE GEOLOGICAL SOCIETY. [DeC. 4, 



between southern Europe and the Antilles, and probably with the 

 Pacific, was kept up during the deposit of the Crag, and even into the 

 Glacial period. Few facts in the natural history of late Tertiary 

 times are more interesting than the dispersion of the Crag Mollusca ; 

 and the discovery of numerous species of them living on the west, 

 Pacific coast of Xorth America, and of some in the West Indies, is 

 as important as that of the offshoots of the Mediterranean algae, 

 which do not exist to the north of Florida, but which flourish in the 

 neighbourhood of the Coral-tracts of the northern part of the Ca- 

 ribbean Sea, constituting one -third part of the marine flora. The 

 recent Coral-fauna of the Caribbean Sea has several genera with a 

 very decided modern Mediterranean facies ; it has none of the 

 commonest genera of the Pacific Coral-fauna ; and it has been already 

 observed that the distinction between the Miocene Coral-forms of the 

 West Indies and those now existing there is very great. 



It is most probable therefore that with the gradual elevation of 

 the vast tracts of reef and of sea-bottom in the Caribbean Sea, and 

 on all sides of it except the east, there must have been such altera- 

 tions in the general contour of the sea-shores, such modifications of 

 land-drainage, and so many adverse external conditions, that the 

 species of solitary Corals especially must have suffered rapid extinc- 

 tion, and the reef-species must have dwindled down and given 

 way to the hardy Porites and Madrepora so characteristic of the 

 present reefs of the Antilles. 



The whole of the Madreporaria are incapable of any other kind of 

 migration than by the dispersion of ova by currents in the sea. The 

 ova are ciliated, and readily attach themselves to substances ; but 

 they require for their growth and development the same external con- 

 ditions as the mature forms. The external conditions have been so 

 fully described by Darwin, Dana, and others, that it is only necessary 

 to observe that the conditions are so peculiar that the discovery of 

 unrolled Madreporaria in fossiliferous deposits, at once satisfies the 

 geologist that the peculiar physical state of things was there present. 

 Yery slight alterations in the physical geography of a Coral- tract in- 

 terfere with Coral-growth, and destroy it if they are persistent. The 

 value of the evidence afforded by fossil Corals is therefore great ; and 

 in estimating it the fact must be considered that several genera of 

 the Gasteropoda, many species of perforating MoUusca, and several 

 genera of fishes depend upon Coral-life for their existence. 



The Orinoco drains a vast Tertiary region, and shuts in the Coral- 

 life of the Caribbean on the south ; for no corals can Hve near its 

 waters ; the Florida reefs consist of few species, and the corals of the 

 Bermudas are the most northerly Madreporaria ; and there are no 

 reefs in the Atlantic. It therefore happens that the species of the 

 present Caribbean Sea are singularly localized ; and this separation 

 from the influences of the Paciffc Coral-tracts dates principally from 

 the commencement of the upheaval of the Miocene deposits of the 

 Isthmus, although it is probable that the isthmus did not become 

 complete until during the Pliocene age. There is some reason in 

 the conjecture that the upheaval of the whole of the Antilles was 



