THE FOEAMINIFERA 



devoid of these organisms, and a little geological 

 knowledge is requisite to secure a suitable piece from 

 the proper horizon in the chalk series. 



The majority of the Foraminifera are of marine 

 habit, although many such species are capable of living 

 far up the estuaries of rivers, provided the water is 

 brackish and well within the reach of tidal influence ; 

 but under these conditions they are as a rule thin- 

 shelled, chitinous, or 

 dwarfed modifications of 

 the more robust marine 

 types. A few forms also 

 are exclusively of fresh- 

 water habit. 



The shells of Forami- 

 nifera are usually micro- 

 scopic, averaging from 

 about 2^3- inch down to 

 -j-A-,7 inch (1 to "25 mm.), 

 but there are certain 

 genera which attain a 

 comparatively large size, such as Orhifolites, wdiich 

 sometimes reaches a diameter of j% inch (1'75 cm.) ; 

 Nunnnulites, with a diameter of 4^ inches (over 11 

 cm.), and Cf/clocli/jjeus, 2J inches (5'7 cm.) There 

 are also other genera, such as Carpenteria and Pohj- 

 trema, which by their accumulative and encrust- 

 ing growth form large masses of shell material, often 

 reaching 5 inches in lateral expanse and an inch and 

 a half to two inches in thickness. 



Fig. 5. — Foraminifera washed 

 FROM Chalk-rock near Dun- 

 stable. X 36. Original. 



