236 GEOGEAPHICAL AND GEOLOGICAL DISTRIBUTION. 



eleven of the southern genera are wanting in the north, and ten of 

 the northern genera in the south. 



The relation of the pelagic or surface fauna to that found in the 

 bottom deposits of the sea, which has given rise to much diversity 

 of opinion among naturalists, has had much light thrown upon it 

 by the recent deep-sea dredging expeditions. It appears now 

 practically certain that a vast majority, probably ninety-eight or 

 ninety-nine per cent., of known foraminiferal forms, including all 

 of the porcellanous and arenaceous groups, are permanent inhabit- 

 ants of the oceanic floor, being endowed with no swimming or 

 floating powers. The surface and mid-water forms are limited to 

 some eight or nine genera and twenty species," many or most of 

 which (species of Globigerina, Orbulina, Spheroidina, PuUenia, 

 Pulvinulina) are identical with forms living in the lowest oceanic 

 layers and found in the Globigerina ooze, to the formation of which 

 they doubtless contribute very extensively through their prodigious 

 development. 



In their importance as rock constituents, the Foraminifera stand 

 second to no other group of organisms; in their geological develop- 

 ment they are probably coeval with the entire period during which 

 the sedimentary deposits were being formed, although it is not 

 till beyond the middle of the Paleozoic series that their un- 

 equivocal remains are met with in any abundance. Barring the 

 much-disputed Eozoon, whose inorganic character may now be 

 accepted without much hesitation, the earliest indication that we 

 possess of this class of animals is in the Lower Silvuian rocks, where 

 thus far but a single genuSj Baccamina, has been positively deter- 

 mined. Two species of Lagena occur in the Upper Silurian of Eng- 

 land. There still exists too much uncertainty relative to the forms 

 known as Receptaculites, Ischadites, and their allies, to permit of 

 their being absolutely classed with the Foraminifera, and likewise 

 in the case of the various structures which have been referred by 

 Ehrenberg to the Silurian of Russia. This paucity in the Silurian 

 rocks, and not less so in the succeeding Devonian, and their com- 

 plete absence from the Cambrian, is not a little surprising, but may 

 perhaps be explained on the hypothesis that the earlier members 

 of the class were in the main devoid of tests, or of such parts as 

 could be readily preserved in a fossil state. The marked develop- 

 ment of the genus Pusulina in the Carboniferous rocks is one of 



