26 PROTOZOA. FORAMINIFERA 



tropical and sub-tropical regions. In the English Eocene the genus 

 is found in the Barton and the Bracklesham Beds. Ex. N. Icevigatus, 

 Bracklesham Beds. 



Operculina. Similar to Nummulites, but whorls fewer and 

 rapidly enlarging, all visible externally ; each of the earlier whorls 

 partly encloses the preceding one. Cretaceous to present day. 

 Ex. 0. complanata, Miocene. 



Orbitoides. Test lenticular or discoidal, composed of a 

 median layer of rectangular chambers arranged in concentric rings 

 which are often incomplete ; the chambers of adjacent rings com- 

 municate by oblique passages. Above and below this layer are 

 numerous layers of smaller chambers ; these chambers are flattened 

 and irregular in form, placed one above the other in piles, and 

 arranged more or less concentrically. The test is minutely per- 

 forated, and canals traverse the septa and marginal cord as in 

 Nummulites ; the septa are also formed of two lamellse. Chalk to 

 Miocene ; chiefly Eocene. Ex. 0. papyracea, Eocene. 



Distribution of the Foraminifera. 



The majority of the Foraminifera are marine, most of 

 them living on the sea-bottom. A few however, as for 

 instance Globigerina, exist at or near the surface in the 

 open ocean, and these are very important on account of 

 their abundance. The distribution of the Foraminifera 

 which live in the open ocean, as well as those found in 

 shallow water, is influenced largely by temperature ; the 

 former are more numerous in the warm ocean-currents 

 than in colder water, whilst the species of the latter often 

 have their range determined by temperature. 



The Foraminifera found in the Palaeozoic deposits are 

 mainly vitreous and arenaceous forms. They appear first 

 in the Lower Cambrian rocks, but are comparatively rare 

 until the Carboniferous, in which some beds are formed 



