DISTEIBUTIOlir or FOKAMIIflPEKA. 235 



perate genera are represented in the tropical and sub-tropical seas. 

 The number of generic forms thus made peculiar to the tropics is 

 about twelve. That all the northern genera should also be found 

 in the central waters is not specially surprising, seeing that the 

 same conditions of temperature which prevail over the one region 

 in the surface or shallow waters are met with in the other in the 

 deeper layers, or upon the oceanic floor. The numerical relation 

 existing between species and genera does not appear to vary accord- 

 ing to any known law of distribution. Many of the genera are 

 most abundantly represented in specific types in the tropics, while, 

 again, others attain their maximum development in the regions of 

 high latitude. Butschli recognises ninety-nine species of Forami- 

 nifera calcarea from the Arctic province, one hundred and eighty- 

 five from the north temperate, and one hundred and ninety- eight 

 from the Mediterranean, an estimate which makes the Arctic prov- 

 ince slightly deficient as compared vpith either of the other provinces, 

 and the north temperate slightly in excess of the Mediterranean. 

 A more thorough investigation of the Mediterranean waters, how- 

 ever, will doubtless increase the number of forms occurring there 

 very considerably, seeing that upwards of one hundred and sixty 

 species have been found on the British coast alone. Despite the 

 seeming diversity given to the character of the Arctic foraminiferal 

 fauna by the ninety-nine or more * species occurring there, the 

 fauna is strictly a uniform one, the vast mass, about ninety-five per 

 cent., of its component material being made up of only a very 

 limited number of species — Globigerina bulloides, Cassidulina laevi- 

 gata, C. crassa, Polystomella striatopunctata, &c. — a feature in a 

 measure also distinguishing the foraminiferal accumulation of the 

 deeper parts of the sea, the Globigerina ooze, in which the indi- 

 viduals of Globigerina, Pulvinulina, Orbulina, Spheroidina, and 

 Pullenia, especially of the former, preponderate to a very marked 

 degree. As in the case of other marine animal groups, the forami- 

 niferal fauna of the high north is most intimately related to its 

 antipodal fauna of the south. Of a total of fifty-three genera and 

 one hundred and eighty-nine species occurring in both the South- 

 ern (below the fiftieth parallel of latitude) and Arctic oceans, thirty- 

 two genera and sixty species are held in common by both areas ; " 



* Brady, in his report on tlio " Cliallenger" expedition, recognises aU in 

 all one hundred and eleven species. 



