16 BULLETIN 71, UNITED STATES NATIONAL MUSEUM. 



In regard to the first of these conditions it may in general be said 

 that there are almost no calcareous Foraminifera to be found in depths 

 greater than 2,500 fathoms. There are certain exceptions to this, 

 but the number of such exceptions is very small. In greater depths 

 than this there are to be found certain species of the arenaceous 

 Foraminifera, which have been found at the greatest depths that 

 dredgings have been made. The reason for the practical disappearance 

 of the calcareous forms below the 2,500-fathom line has not yet been 

 definitely settled. It has been thought that the great pressure at 

 that depth has made their dissolution much easier than at shallower 

 depths, but this has not been conclusively demonstrated. 



Besides the disappearance of certain groups of forms through 

 dissolution, there are found to be rather definite limits to the range in 

 depth of any particular species when a considerable series may be 

 had for study. Many species are found only in very shallow water, 

 such as Alveolina, Amj^histegina, etc. They may be found occa- 

 sionally at considerable depths in the vicinity of oceanic islands, 

 such as Guam, where the currents carry them into deeper water, but 

 even in such cases the tests show the effects of wave action and are 

 as a rule tests which were apparently empty before they were carried 

 out to this depth. In the neighborhood of such islands very strange 

 mixtures of deep-water with shallow-water species may be found, 

 and mingled with the other two are often many of the typical species 

 of the globigerina ooze. 



Bottom temperature has in some cases an apparent influence upon 

 the distribution of certain species. For example, certain species of 

 the arenaceous group which are ordinarily found in deep and cold 

 water are found in* comparatively shallow water in such regions as 

 that of the Okhotsk Sea, where the bottom temperatures are very low. 

 Apparently in such cases their presence at such depths is to be 

 explained upon the basis of temperature. In a similar manner there 

 are a certain number of species which seem to thrive only in the warm 

 water of the tropics and are for the most part found associated with 

 the other species of animals characteristic of coral reefs. 



Ocean currents play an important part in the distribution of species 

 in two ways — (1) by determining in part the distribution of ocean 

 temperatures, and therefore in this way limiting or extending the 

 range of certain species; and also (2) through their position and 

 direction determining the distribution of the pelagic tests on the 

 ocean floor. This will be shown in dealing with distribution of the 

 various species of Globigerina. 



With these various factors it is not surprising that definite faunas 

 should exist in different parts of so large and varied a region as that 

 of the North Pacific. In this study a number of rather definite faunas 

 have been met with which may be clearly separated. In general the 



