138 THE FORAM1NIFERA 



period when the Carboniferous and Permian rocks were deposited, 

 had undergone elongation in the direction of the spiral axis and 

 been differentiated into a series of forms biconvex, obovate, 

 spheroidal, and fusiform closely comparable with those assumed 

 by the species of Alveolina of the Tertiary period and the present 

 day (cp. Figs. 42 and 59). 1 



Distribution. From this point of view the Foraminifera may be 

 divided into two classes the attached or bottom-living and pelagic 

 forms. While by far the greater number of genera and species 

 belong to the former, the numbers of individuals of the latter are 

 enormously great. 



The Pelagic Foraminifera belong to the genera Globigerina (with 

 its connected form Orbulina), Hastigerina, Pullenia, SpJiaeroidina, 

 and Candeina forming the order Globigerinidea and Pulvinulina 

 and Cymbalopora among the Rotalidea. The pelagic habit of these 

 forms, though it had previously been recognised by M 'Donald and 

 Major Owen, was first clearly established by the naturalists of the 

 Challenger. 



The species which are found at the surface extend down to 

 considerable depths, but whether they may actually live on the 

 bottom of the ocean is still, in spite of much discussion, undecided. 

 They congregate at the surface at night, and partially withdraw 

 from it during the day. 



It is in the equatorial and temperate regions of the ocean that 

 they most abound, the pelagic forms being represented in the arctic 

 and antarctic seas by small species of Globigerina : G. pachyderma in 

 the former, G. dutertrei in the latter. 



Beneath this equatorial belt of warm water and its northern 

 offset, the Gulf Stream, the empty tests of the pelagic Foramini- 

 fera constitute the main portion of the " Globigerina ooze," which 

 forms the ocean floor down to a depth of 3000 fathoms. As this 

 limit is approached the thinner tests disappear, and beyond it all 

 calcareous constituents are removed by solution. 2 



The species of bottom-living Foraminifera have, on the whole, 

 a very wide distribution. Some are cosmopolitan, ranging from 

 arctic to tropical waters and from shore pools to the bottom of 

 the great oceans. A large proportion of the genera, however, are 

 restricted by depth and temperature. The shallow littoral waters 

 of the tropics contain an abundant fauna, most of the members of 

 which do not extend to colder seas. On the other hand, genera 



1 The Lituolidea are described as " isomorphoua " with various calcareous genera, 

 but it is far from certain that the similarity in form does not depend on true affinity, 

 in which case the term is not strictly applicable. Loftusia, among the Lituolidea, 

 has a fusiform test, externally resembling the more elongated forms of the Fusulina 

 and Alveolina series. 



a Cp. Sir John Murray, "On the Distribution of the Pelagic Foraminifera," etc. 

 Natural Science, July 1897, pp. 17-27. 



