370 GEOGKAPHICAL XSD GEOLOGICAL DISTKIBCTION. 



of the Paleozoic series tends to link the different members to- 

 gether. 



The earliest great difiEerentiation in the ancient gasteropod fauna 

 is seen in the Jurassic deposits, where a host of new forms, especially 

 of the Siphonata (of the families Cerithiidse, Nerinseidae, Apor- 

 rhaidae, Strombidae, Buccinidse, Purpuridse, Columbellidae, &c.), 

 are for the first time introduced. One or two of these families 

 appear to have had their representatives already in the Trias, but' 

 they were there of insignificant import. It is not, however, until the 

 Cretaceous period that many of the more distinctive of the modern 

 families (Cyprseidae, Cassididae, Ficulidae, Tritonidae, Muricidae, 

 Volutidae, Olividae, Cancellaridse, Terebridae, Pleurotomidae, and 

 Conidae) appear, and of these a fair proportion of the genera date 

 back only to the Eocene period. No recent species is recognised 

 as extending back beyond the Tertiary series, and even in the 

 Eocene the proportion of living to extinct forms is very slight, 

 averaging not more than three to five per cent. ; indeed, it is ques- 

 tionable whether any of the early Tertiary species can be identified 

 with recent forms. The same is also possibly true of the Oligocene, 

 but in the Miocene the percentage ranges as high as thirty-five, 

 and in the Pliocene to seventy-five or more. Practically, all the 

 Post-Pliocene forms are still living. It is impossible to arrive at 

 any absolute estimate of the number of species occurring in each 

 formation. Bigsby " enumerates some seven hundred to eight hun- 

 dred species as belonging to each of the Silurian, Devonian, and 

 Carboniferous periods, or very nearly that which is given by Zittel " 

 for the Jurassic forms ; the Permian, which is deficient in nearly 

 all forms of life, has but about thirty. The Eocene-Oligocene 

 Paris basin contains, according to Deshayes, upwards of eighteen 

 hundred species, or more than double the number of the entire 

 Eocene shell fauna of the Eastern and Southern United States. 

 The Miocene basin of Vienna holds upwards of four hundred species 

 of Prosobranchiata. 



There are but very few truly cosmopolitan species of fossil 

 Gasteropoda, although broad distribution was much more marked 

 in the early periods of the earth's history than now. Thus, while 

 from the American Tertiaries only a very insignificant number of 

 forms could be selected which might in any way be correlated 

 with contemporary European species, and but a fairly representative 



