1900] MICROSCOPICAL JOURNAL. 215 



As regards their distribution in space, the Radiolarians 

 are exclusively marine, and are found in all seas and all 

 depths. I have seen one Dictyocha fibula in the brackish 

 water where it became fresh in the Passaic River at Belle- 

 ville, N. J. But they are commonly floating organisms, 

 and they are often present in enormous numbers so that 

 they can be easily got and studied, but in the warm seas 

 I have not found them except in the case mentioned above 

 in this latitude. Many are pelagic, and inhabit the sur- 

 face waters of all oceans soever. Others are abyssal and 

 are confined to great depths in the sea ; while others 

 again, are "zonarial" and confined to particular bathymet- 

 rical horizons between the surface and the bottom. Over 

 large areas of the deep sea, principally at depths of from 

 2,000 to over 4,000 fathoms, the bottom is found to be 

 covered with extensive deposits, hence called "Radiola- 

 rian ooze." This deposit is a siliceous mud, with little 

 calcareous matter, which is composed more or less large- 

 ly of various Radiolarians. The skeletons of Radiola- 

 rians are, however, also present, in smaller or greater 

 numbers in many of the marine deposits which are form- 

 ed at comparatively limited depths. 



As regards their distribution in time, Radiolarians are 

 abundantly represented by fossil forms, which are now 

 known to have high antiquity. Their past history is the 

 Acantharia in which the skeleton is not siliceous but com- 

 posed of acanthin and they are wholly unknown for that 

 reason in a fossil condition. This is likewise the case with 

 the whole groop of Phseodaria, in which the skeleton is 

 composed of silicate of carbon, with the single exception 

 of the small group represented by Dictyocha audits allies 

 in which the skeleton is purely siliceous. This genus be- 

 longs to the Upper Chalk, and is represented in Tertiary 

 deposits and in recent seas. Until recently Radiolarians 

 have only been detected in the fossil condition in depos- 

 its of Kainozoic and Mesozoic age ; and our knowledge of 



