398 University of California Puhlications in Zoology [Vol. 18 



PAGE 



Paired areas of change of plate patterns 444 



Completeness of the series of variations 446 



Geometrical patterns represented 44(3 



Combinations of patterns 446 



Variations in the length of critical sutures 448 



Significance of variation in suture length 450 



Mutations 456 



Possible influence of the environment 457 



C. Conclusions 458 



D. Bibliography 466 



E. Plates and explanations of plates 472 



A. INTRODUCTION 



Purpose and Scope of this Paper 



The purpose of this paper is to describe certain structural char- 

 acteristics of the theca of the genus, Peridinium, which have been 

 used as a basis for the classification of these organisms, but which 

 seem to have a much more fundamental significance, possibly por- 

 traying the basis of natural relationships and of speciation. The 

 theca is constructed with considerable regularity and simplicitj^ in 

 each species of the group of dinoflagellates to which this genus belongs. 

 The structure of the theca for different species seems to be based upon 

 the modification of a given number of parts according to a system 

 which may be extended to the genera of the greater part of the group. 

 Not only in Peridinium but also in several other genera of the dino- 

 flagellates the regions of comparative permanency of skeletal structure 

 and the regions of variability are usually definitely located. These 

 conditions give the problem of variability in this group a certain wel- 

 come discreetness. 



The group, Dinoflagellata, is also a w^idely distributed group, 

 usuall}' abundant wherever found. It contains certain characteristic 

 cold and warm water forms as well as certain species which seem to 

 be ubiquitous. Material is thus readily accessible and may be secured 

 from a great variety' of environments. 



]\Ioreover, the medium in which these organisms live presents con- 

 ditions not encountered in the environment of land organisms which 

 have usually been employed for variational studies. In the sea, of 

 all places on earth, life may be found under conditions which change 



