214 IT'- H. BALL 



winds from a quarter which led to the forcing to the eastward of the 

 Gulf Stream water, and to the influx of much colder water from the 

 Polar current into the area thus vacated, was believed to be respon- 

 sible for the almost total extermination of these fishes, which were 

 found floating dead and apparently uninjured in millions on the 

 surface of the sea, by navigators bound into New York and adjacent 

 ports. 



This temperature change which lasted at most for a few weeks 

 would probably have had no effect whatever on the adult larger 

 invertebrates of the same area, though to any of their larval young it 

 might well have proved fatal. Another season would replace these, 

 but the restocking of the fauna with "tilefish," which finally took 

 place, required many years. 



A statement of the factors which are regarded as modifying 

 existing marine invertebrate faunas will put the student in possession 

 of the chief factors which may have affected analogous faunas during 

 past geologic time. My point of view is that afforded by a knowledge 

 of conditions affecting molluscan life. 



Census of species. — From a discussion too long to quote here in 

 full/ I have drawn the following conclusions: That the part of the 

 average mollusk-fauna which is capable of leaving traces in the shape 

 of fossils, under conditions not greatly differing from those of the 

 present day, in a region where the temperature of the sea ranges 

 during the coldest winter month between 32° and 40° F. (which 

 might be called boreal) , would comprise about 250 species. In case 

 the temperature ranged between 40° and 60° {cool temperate) about 

 400 species might be expected. With a range between 60° and 70° 

 {warm temperate) we should find about 500 species, and in the tropical 

 zone (70° to 80° F.) not less than 600 species; and in specially fa- 

 vorable localities of the tropics nearly twice as many. 



Learning from the characteristic genera what zone of temperature 

 a given fauna may have belonged to, we can with confidence predict 

 approximately the number of species which it will prove to contain 

 when fully explored. Of course in a single locality where the char- 

 acteristic situs is exclusively mud. or rock, or fine sand, only a certain 



I Bull. U. S. Geological Survey, No. 84, Correlation Papers, Neocene, 1892, pp. 



2=;-2S. 



