/ 
WV ROTHESTS OF CAUSE OF GLACIAL PERIODS i. 
There are not more than twenty or twenty-two species of pelagic Forami- 
nifera, yet so numerous are the individuals of the species that they usually 
make up over go per cent. of the carbonate of lime present in the calcareous 
oozes of the abysmal regions of the ocean... . 
The bottom-loving Foraminifera—those belonging to the Benthos—avre 
more abundant in the shallow water, than in the deep-sea deposits, and 
occasionally a single species may occur in such abundance in shallow depths 
in some regions as to make up the greater part of a deposit. . . 
The presence of large numbers of Pteropod and Heteropod shells indicates 
tropical or subtropical regions, and relatively shallow depths. Abundance of 
the shells of pelagic Forminifera indicates the same regions, but when found 
without the shells of pelagic mollusks they indicate a greater depth than 
when these latter are present. . .: The presence or absence, and the size 
of Rhabdoliths, Coccoliths and Coccospheres gzve zmportant indications as to 
latitude and depth —the first predominating in tropical regions, the two latter 
being better developed in temperate regions, and a// disappear from the 
deposits as the polar waters are approached. 
*A large number of these pelagic mollusks (Pteropod and Heteropod) 
secrete carbonate of lime shells, and this ts especially the case in tropical 
waters. In folar regions the place of the shelled species is taken, with the 
exception of one or two small species of Limacina, by a shed/-less shecies. 
The shells of the tropical species make up a large part of some tropical and 
subtropical deposits from moderate depths, in which there is a relatively 
small quantity of land débris. Like the pelagic Foraminifera these pelagic 
Mollusca attain their greatest development in the warm oceanic currents, 
and diminish both in the number of species and the size and mass of the shells 
as the colder currents of the polar regions are approached. 
Reef-forming corals are confined to waters which, through even the 
coldest month, have a mean temperature not below 68° F. Under the equator 
the surface waters in thehotter part of the ocean have the temperature of 
g5° F. in the Pacific, and 83° F. in the Atlantic. The range from 68° to 85° 
is, therefore, not too great for reef-making species." 
An isothermal line crossing the ocean where this winter temperature of 
the sea is experienced, one north of the equator, and another south, bending 
in its course toward or from the equator, wherever the marine currents 
change its position, will include all the growing reefs of the world; and the 
area of waters may be properly called the coral-reef seas. 
Over the sea thus limited coral reefs grow luxuriantly, yet in greatest pro- 
fusion and widest variety through tts hottest portions.* 
I have found no specific statements relative to the dependence 
of common mollusks on temperature, but the enumeration of the 
Dana: Corals and Coralline Islands, p. 83. 
