462 University of Calif oriria Publications in Zoology [Vol.18 



tioii of plates 1" and 7" of this genus. Another factor in this con- 

 nection may be the small size of most fresh water forms, due perhaps 

 to the lack of buoyant power of the fresh water which, prohibiting any 

 great disparity between surface and volume, may set a limit upon 

 expansion. 



16. There are then four variable regions which may vary either 

 independently of each other or in dorsal or ventral couples and which 

 may combine their variations with each other in a total of sixteen 

 possible combinations of which at least nine are represented in the 

 existing species of this genus. 



17. The significance of this complete series of combinations of 

 approximately symmetrical patterns shows not only the pressure of 

 the influence for variation and the great extent to which this group 

 of organisms seems to have expanded in all possible directions of 

 change, but even more significantly demonstrates the progressive 

 sequence in which these changes of pattern must have occurred. 



18. Each of the symmetrical patterns seems to represent a stage of 

 stability in this process of progressive change, a period when the 

 skeletal elements and the forces playing upon them are in a state of 

 approximate equilibrium. Less frequently do the asymmetrical pat- 

 terns seem to be so stable, though that of Metaperidimmn for the 

 ventral plates is found consistently in a considerable number of species. 

 The asymmetrical patterns of the dorsal side often seem, however, to 

 be concomitants of extensive changes of the physical condition of the 

 environment, and do not pervade a great portion of the genus. 



19. A significant consideration with respect to the combination 

 of symmetrical and asymmetrical patterns appears from the correla- 

 tion of the asymmetrical dorsal patterns only with the asymmetrical 

 ventral pattern. The principal of symmetry seems to be the control- 

 ling one in this genus and the species having asymmetrical patterns 

 are apparently less able, in most cases, to persist than are those having 

 symmetrical patterns. 



The symmetry referred to in examining the various plate patterns 

 of Pcridinium is only an approximate and not a perfect symmetry, 

 it being apparently a fundamental character of Dinoflagellata that 

 the body is still more or less asymmetrical. However, in view of the 

 extreme asymmetry displayed by related genera in the development 

 of antapical horns and in the position for the introduction of the acces- 

 sory plates, where these occur, the approximate symmetry maintained 

 by the skeletal structures in most of the species of Peridinium attracts 

 attention. 



