994 PRINCIPLES OF STRATIGRAPHY 



delicate larvse, and, as soon as the old conditions are reestablished, 

 this fauna will again appear with countless individuals. This ex- 

 plains the sudden reappearance in later strata of the fauna of an 

 earlier bed, even though it was absent from the intervening strata. 

 From a stratigraphic point of view the meroplankton is of vast 

 importance, for to it is due the wide dispersal of the benthonic 

 organisms, which of all marine life forms are the best indices 

 of the physical conditions of the sea bottom. It is during the larval 

 period that benthonic marine invertebrates spread in all directions 

 from their center of occupancy, and that they have an opportunity 

 to enter and occupy distant regions. 



3. Pscitdoplankton. This term was introduced by Schiitt (8) 

 for such organisms which, like the Sargassum, are normally or in 

 early life benthonic, but continue their later existence as plankton. 

 Walther has extended the meaning of the term so as to include 

 those organisms which are carried about by floating objects, to 

 which they are either attached as sedentary benthos or which serve 

 them as a substratum on which they lead a vagrant benthonic 

 existence. This group, however, is distinct, and is separated here 

 under the name of cpiplankton. 



Under pseudoplankton may, however, be included those ben- 

 thonic or nektonic organisms which become planktonic only after 

 death ; i. e., that portion of the plankton which consists of the float- 

 ing parts of dead organisms, such as the leaves and trunks of 

 trees, dead insects and carcasses of vertebrates, and, above all, 

 the shells of molluscs, which, on the death of the animal, are dis- 

 tributed more or less widely, according to the nature of the shell. 

 Thus the shell of Spirula is widely distributed and embedded in 

 contemporaneous sediments where the organism never lived, and 

 leaves of terrestrial plants are found in sediments far from land. 

 Tree trunks from tropical America are carried to the northeast 

 coast of Iceland, where they become embedded in contemporaneous 

 sediments. 



4. Epiplankton. This term (from eTti, upon) was proposed 

 (Grabau-3) for those organisms which live upon a floating sub- 

 stratum, to which they are either attached or upon which they lead 

 a vagrant existence. The substratum may be holoplanktonic or 

 pseudoplanktonic, the latter being the most frequent. Examples of 

 epiplankton are the algje, hydroids, and bryozoans attached to the 

 floating Sargassum and other alg?e ; and the Crustacea, molluscs, 

 and other animals living among their branches ; and the organisms 

 attached to floating tree trunks and other pseudoplankton. 



A large number of algse, especially the shallow- water forms, 



