THE PALEONTOLOGIC RECORD 



century this edible European gastropod was introduced at Halifax, 

 Nova Scotia, and in 50 years attained the Delaware Bay and north to 

 Labrador. Taking this dispersion as the basis for calculating f aunal mi- 

 grations, we learn that they may spread 500 miles, while one sixteenth 

 of an inch of average sediment is depositing, or 8,000 miles during 

 the time of one foot of sedimentary accumulation. If, therefore, Paleo- 

 zoic faunas migrated " only one fiftieth as fast as this living shell, then 

 we may reasonably assert essential contemporaneity for stratigraphic 

 correlations extending entirely across the continent." We have here 

 an explanation for the apparently sudden distribution of the Ordovicic 

 brachipod Rhynchotrema capax, that everywhere holds an identical 

 geologic horizon from Anticosti to the Big Horns and from El Paso, 

 Texas, to Arctic Alaska. Spirifer hungerfordi spreads during the first 

 half of Upper Devonic time from the Urals to Iowa, and another 

 brachiopod, String ocephalus burtoni, migrates during the last third of 

 Middle Devonic time from western Europe to Manitoba. 



The life of the present seas extends from the strand-line to the 

 deepest abj^ss, but by|ar the greatest quantity and variety lives in the 

 upper sunlight, photic or diaphanous region. Photographically the 

 light of the sun is detectable in exceptionally clear-water tropical seas 

 to a depth of about 2,000 feet, but Johnstone places the average depth 

 for all waters at 650 feet, beyond which there is more or less of total 

 darkness, the aphotic realm. 



Sunlight is the first essential for the existence of life. Where it 

 penetrates, there plant life is possible, and this life is the substratum 

 on which all animal life is ultimately dependent for food. Near the 

 surface of the sea lives the plankton, sometimes referred to as the 

 "pastures of the sea" and compared with the "grass of the fields." 

 Most of this plankton consists of diatoms that at present are by far 

 more prolific in the cooler polar waters. At times of greatest abun- 

 dance in Kiel Bay as many as 200 of these " jewels of the plant world " 

 are contained in a drop of water, and in the Antarctic seas there is 

 an area of ten and one half million square miles where diatom ooze is 

 accumulating. They are the principal food supply for most of the ses- 

 sile benthos, or bottom life, among which the mollusca and brachiopods 

 are of the greatest importance in paleogeography. 



Geologic deposits rich in diatoms are sometimes regarded as those 

 of the deep sea, at least as of deeper waters than those of continental 

 seas. The English Carbonic deposits, rich in diatoms, have a fauna 

 whose species are all of the shallow water kinds. The vast Miocene 

 diatom deposits of California, described by Arnold, have living bottom 

 types of foraminifera that, according to Bagg, do not indicate a depth 

 of over 500 fathoms. 



From the present distribution of marine life we learn that the 



