320 PALEONTOLOGY 



organisms. A few genera of Foraminifera attain a much 

 greater size, up to a maximum of perhaps 80 mm., and 

 these giant forms are of value as indices of age, though 

 unfortunately their geographical range is limited. 



The Radiolaria range from possibly pre- Cambrian to 

 the present day : they are divided into two main sub- 

 orders Spumellaria, spherical or discoidal, with the range 

 of the order, and Nassellaria, of various forms, but always 

 with dissimilar ends, dating from the Devonian only. 

 Beds of Radiolarian chert are known in the Ordovician 

 of South Scotland, the Devonian of New South Wales, 

 the Carboniferous of Devon and Cornwall, and the 

 Mesozoic of the Alps. These beds have very generally ' 

 been taken as analogous to the modern Radiolarian ooze 

 of the greatest depths of the oceans ; but in the case of 

 the British Lower Carboniferous cherts their shallow- 

 water character seems now decisively proved. On the 

 other hand, the Miocene Radiolarian earths (not con- 

 solidated into chert) of Barbados and other West Indian 

 islands are probably of deep-sea origin. 



The Foraminifera form a shell which is nearly always 

 chambered, the chambers being arranged sometimes in a 

 straight line, sometimes in a zig-zag, but most frequently 

 in some sort of spiral. They were at one time taken for 

 cephalopods, but the resemblance in the shells of the 

 two groups is only homoeomorphy of the most general 

 sort. There are no gas-chambers in a foraminifer, the 

 cell-protoplasm filling all the chambers as well as ex- 

 tending to a greater or less extent outside the shell ; this 

 it does through a well-defined aperture (much smaller 



