THE RADIOLARIA 113 



in Atlantic and Antarctic waters (1) the majority of Radiolaria 

 occur not deeper than 400 metres; that the Collodaria are em- 

 phatically surface forms characteristic of the top stratum (0-50 m.) ; 

 (2) that in the next stratum below this (50-400) the great develop- 

 ment of Radiolarian, as also of diatomaceous, life occurs. Here the 

 majority of Acantharia, many Spumellaria, and many Phaeodaria, 

 e.g. Challengeridae, occur; (3) that in the still deeper water, 400- 

 1000 metres, a still richer Phaeodarian fauna and a few Acantharia 

 are met with, and that beyond this a few remarkable forms range 

 down to 5000 metres. The vertical distribution of the Nassellaria 

 is not yet adequately known, but it probably follows much the 

 same lines as that of the Phaeodaria. 



JB t Horizontal. The distribution of the class is extremely wide, 

 as is readily understood from their dispersal by the great oceanic 

 currents. Some forms are panplanktonic, e.g. Aulacantha ; some 

 are bipolar; many are emphatically warm -water forms; others as 

 characteristically follow cold currents. Such considerations enable 

 us to understand the varying depths at which the same form may 

 occur as its chosen current occupies now a deeper, now a more 

 superficial position in the ocean. The greatest variety of species 

 is met with in equatorial waters, and this fulness extends in 

 diminishing variety north and south for some forty degrees. Then 

 there follows, at least in the northerly direction, as in the case of 

 many othej- pelagic orders, a barren zone, and finally Arctic waters 

 show a Radiolarian fauna that is rich in individuals though poor in 

 variety, and is apparently greatly inferior to that of Antarctic 

 (Hacker). This mode of distribution explains the comparative 

 poverty of the British Radiolarian fauna. Though the lack of 

 research makes reserve necessary, it seems certain that these waters 

 of the west and north-east coasts of Britain contain only a casual 

 Thalassicolla and a few Acanthometrida, Sphaerellaria, and Phaeo- 

 daria, outliers and stragglers of the rich Gulf Stream fauna. 

 The great northern host passes by the Faroes and off the Hebrides, 

 as the lists, pp. 144-151, show, and in those waters the researches 

 of Murray, Fowler, and Wolfenden have revealed a number of 

 interesting forms. 



The deposits formed by the accumulation of Radiolarian 

 skeletons constitute a well-known element in the composition of 

 littoral and deep-sea Globigerina ooze and of red clay. They make 

 up certain of the clays, marls, and pumices found in the Miocene 

 deposits of Barbadoes, the Nicobar Islands, and on both sides of the 

 Mediterranean, as at Oran and Tripoli. Siliceous organic rocks 

 of Palaeozoic and of Mesozoic age have been recently discovered in 

 many parts of the world ; and microscopical investigations of these 

 rocks have revealed an unsuspected wealth of Radiolaria in them. 

 From the Cambrian age onwards, however, the families and even 



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