DTSTEIBUTION OF MOLLUSCA IN TIME. 421 



vered similar beds near Quebec, 50—200 feet above the River St. Lawrence, 

 containing an assemblage of shells entirely arctic in character ; whereas in 

 the present gulf he obtained an admixture of the American representatives of 

 Lusitanian types, Mesodesma, Periploma, Petricola, Crepidida. 



The glacial deposits of the northern hemisphere extend about 15° south 

 of the line of " northern limit of trees ;" but this comparatively recent exten- 

 sion of the Arctic ocean does not appear to have much iufluenced, if it ever 

 invaded, the inland basin of the Aralo-Caspian, which contains only one 

 species common to the White Sea, Cardium edule, var. rusticum.^ 



The older pliocene period is represented in England by the Coralline 

 Cnuj, a deposit containing 340 species of shells. Of these 73 are living 

 British species, but (with two or three exceptions) they are such as range 

 south of Britain. (Forbes) The remainder are extinct^ or living only to the 

 south, especially in the Lusitanian province; e.g. Fossarus sulcatm, Luci- 

 no2)sis LajonJcairii, Chama gri/phoides, and species of Cassidaria, Cleodora, 

 Sigaretus, Terebra, Columbella and Pyramidella. It also contains a few 

 forms belonging to an earlier age,— a Pholadomya, a true Pijrula, a Lingula, 

 and a large Volata, resembling the Magellanic species. 



The shells of the newer tertiaries are always identical, at la^^i genericalli/, 

 with those of the nearest coasts. Thus, in Patagonia, are found species of 

 Tro2o7ion, Crepidula, Monoceros, Pseiidoliva, Vohita, Olvm, Crassatella, and 

 Solenella. The tertiaries of the United States contain species of Fulgur, 

 Mercenaria and Gnathodon. The miocene shells of St. Domingo appear at 

 first sight to be ail of recent species, but on comparison prove to be mostly 

 distinct. 



The proportion of extinct species in the Pliocene tertiary, varies from 

 1—50 per cent. If a deposit contains more than 50 per cent, of extinct 

 species it is referred to the 3Iiocene period ; and this test is particularly 

 valuable since the modern deposits are often isolated, and frequently no assist- 

 ance can be derived from superposition, or even from identity of species. 



In the Eocene tertiaries we perceive the " dawn" of the present order of 

 things. All, or very nearly all, the species are different, but a large propor- 

 tion of the genera are still existing, though not always in the seas nearest 

 to the localities where they occur fossil. 



Thus in the London clay arc found — Rostellaria, Oliva, AncUlaria, and 

 Vulsella, genera still living in the Red Sea ; and many species of Nautilus, 

 Rimella, Seraphs, Conus, Mifra, Pyrula, Phorus, Liotia, Cardilia, — genera 

 characteristic of the Indian Ocean ; Ci/j)rovula, Ti/j)his and Volutilithes, now 



* Mr. Wm. Hopkins of Cambridge has investigated the causes which may have 

 produced a temporary extension of the Arctic phenomena in Europe; and considers 

 the most efficient and probable cause would be a diversion of the Gulf-stream, which 

 he supposes to have flowed up what is now the valley of the Mississippi.— (Geo/. 

 Journal.) 



