404 University of California Puhlications in Zoology [Vol. 18 



supposed to approximate as nearly as possible early primitive condi- 

 tions under which organic life may have existed, and under which, to 

 judge from the fossil remains of other groups of Protista, Foramini- 

 fera, Radiolaria, and Diatomaceae, for instance, there has been a mini- 

 mum or at least a very slow evolutionary change from the forms of the 

 Carboniferous seas to those of the present ; 



(5) The complications, on the other hand, introduced because of 

 the lack of concrete barriers in the sea, making physiologic barriers of 

 especial importance ; 



(6) Life in an environment without a substratum and one in 

 which the maintenance of an organism under optimum conditions 

 depends upon the continuance of its active functions or the rapid 

 adjustment between surface area and volume of the organism, per- 

 haps necessitating for the pelagic plankton further adaptation by 

 means of resting stages during long periods of severe conditions ; 



(7) The vital importance of minute variations either initiated 

 within the organism or stimvilated by the stress of the environment, 

 and the significance of small measurable variations in any of the 

 physical factors of the environment, illustrating the delicate balance 

 which must be maintained between these organisms and their environ- 

 ment if thev are to continue their existence. 



Systematic Position of Peridinium 



Recent taxonomic arrangement recognizes two orders in the sub- 

 class, Dinoflagellata, of the Mastigophora ; the Adinida containing 

 the genera, Exuviaella and Prorocentrum, in which "the typical 

 peculiarities of Dinoflagellate organization are not fully developed" 

 (Minchin, 1912, p. 278), and the Dinifera, having the typical charac- 

 teristics of the subclass (Minchin, 1912, p. 278; Doflein, 1911, 

 pp. 525-531). 



The order, Dinifera, is in turn regarded as composed of three com- 

 prehensive families, the Gymnodinidae, Peridinidae, and Dino- 

 physidae. The Gymnodinidae are distinguished as being shell-less, or 

 at least without a shell which has been divided into plates and which 

 may be visibly distinguished as a structure set off from the main 

 protoplasmic mass of the body. There is indeed some suggestion that 

 some of the forms now placed in this family may be only incompletely 

 developed forms of the next family, in which the shell characteristic 

 of the adult stage, usually met with, has not been fully developed 



