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 faunal 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, Stringocephalus 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 abyss, but by far 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 t
