STRATIGRAPHIC CLASSIFICATION PALEONTOLOGIC CRITERIA 483 



faunas of the Silurian and earlier systems, are of great value in inter- 

 continental correlations. Unfortunately, however, the occurrences of 

 graptolites are largely confined to narrow channels near the margins of 

 the continents and are therefore only occasionally available in determin- 

 ing ages of deposits in more inland basins. 



CENTERti OF ORIGIN AND DISPERSAL OF FOSSIL MARINE FAUNAS 



Although in the past geologic ages each of the present deep oceanic 

 basins probably was in a greater or smaller degree a center of development 

 and dispersal, comparative studies of Paleozoic faunas in the northern 

 hemisphere suggest grouping these into three major faunal realms, 

 namely, (1) an Atlantic, (2) an Arctic, and (3) a Pacific. When occa- 

 sion offered these seas and their contained faunas invaded the intervening 

 epicontinental depressions to varying extents and in such manner that 

 many of these basins have been alternately occupied by Atlantic and 

 Arctic or by Arctic and Pacific faunas. In rare instances, as in Okla- 

 homa, faunas apparently derived from all three realms are superposed. 



Possibly the continental basins themselves sometimes became centers of 

 origin and dispersal. Indeed, in these basins local "expansional evolu- 

 tion,"^^ a subject that will be discussed in some detail presently, may seem 

 occasionally to mask the invading faunal element that is chiefly to be de- 

 pended on in distant correlations. Obviously, since the indigenous forms, 

 if such they be, consist mostly of minor departures from strong specific 

 stocks, they are often of exceptional value in exact correlation within 

 provincial or more restricted geographic limits. 



Paleozoic Atlantic faunal realm and its subdivisions — The three sulv 

 faunas. — The Paleozoic Atlantic fauna seems to comprise three subfaunas : 

 (1) the north middle Atlantic, ranging approximately between the latitude 

 of Newfoundland on the north and Florida and the Antilles, which vveic 

 occasionally united, on the south; (2) the Gulf of Mexico and the Carib- 

 bean Sea, which basins are thought to have been sometimes divided by 

 land connection between Yucatan and the Antilles, and (3) the Mediter- 

 ranean. As a rule these faunas are readily distinguishable but at certain 

 times, referring more particularly to the American side, the bars appear 

 to have been let down between them. Considerable community between 

 the northern basin and the Gulf of Mexico is indicated toward the close 

 of the Ordovician by faunas found in the Carodoc of Wales and the Cin- 

 cinnatian in southeastern America. Even stronger commingling occurred 

 during the early Devonian, while from the Waverlyan on to the present 



^« T. C. Chamberlln : Journal Geology, vol. 6, pp. 457-459 and pp. 600-608. 



