336 



pseudo-planktonic existence, this barnacle rarely occurring 

 except as attached to floating objects. Many of the animals 

 found on the Sargassum seem to be characteristic of it in its 

 floating condition, not occurring on it in its native haunts. 

 (Ortmann.) Walther has adduced evidence which goes to 

 show conclusively that many of the larger fossil Pentacrini, 

 and perhaps other crinoids, occurred with their stems wound 

 around floating timbers, and he explains the occurrence of 

 these marine animals in fresh-water coal strata as due to 

 stranding in estuaries of species leading a pseudo-plank- 

 tonic existence. 



Bionomic Characteristics of the Classes of Marine 



Invertebrates Important from a Pal^eon- 



tological Point of View. 



Foraminifera,. — The Foraminifera are typically marine 

 organisms, though a considerable number of species have 

 become adapted to brackish water, living in estuaries and 

 near the mouths of streams, while a number of species, com- 

 monly placed in this class, live entirely in fresh water. Their 

 distribution is so great that scarcely any marine sediments 

 a in' wholly free from the shells of these animals. Most Fora- 

 minifera belong to the va-grant benthos, though sedentary ben- 

 thonic forms also occur. Only something over twenty living 

 planktonic species are known, these belonging chiefly to the 

 genera Globigerina, Orbulina, and Pulvinulina (Haeckel), the 

 first predominating. The small number of species is. 

 however, counterbalanced by the enormous number of indi- 

 viduals. The benthonic Foraminifera arc confined chiefly to 

 thf littoral district, where the character of the bottom and 

 t he temperal ure of the water exerts important influences on 

 t lie distribution of these organisms. A muddy fades of the 

 sea bottom seems to be conducive to the existence of a large 

 number of species, but the rocky bottoms are not without 

 their types: while algae and sea*grasses commonly form 

 the home of vast numbers of these organisms. The coa.rse, 



