408 University of California PuMications in Zoology [Vol. 18 



B. MAIN ARGUMENT 



Abstract of Argument 



The cellulose-like shell or skeleton of the genera of Peridinidae, 

 biflagellated Protozoa of the sea, shows a progressive specialization 

 in the increasing number of plates into which the skeleton is divided 

 in the several genera. This process of specialization can be traced in 

 detail in species of Peridinium, one of the principal genera of the 

 family. Among the species of this genus the arrangement of the 

 plates in four regions on the anterior part of the skeleton undergoes 

 definite changes, producing different plate patterns limited in num- 

 ber by the geometric possibilities for the rearrangement of the plates. 

 These plate patterns are not only of great convenience in classifica- 

 tion but because of the fundamental morphologic importance of the 

 skeleton, the arrangement of its elements may be relied upon to show 

 the relationships of many of the species of the genus. As represented 

 in these species, each pattern is distinct, and transitionary stages from 

 one pattern to another are unknown. Exceptional cases, however, 

 have been found of specimens which, while bearing many character- 

 istic marks of a certain species still display a plate pattern different 

 from that typical for the species. 



It is strongly suggested that each pattern represents a condition 

 of skeletal equilibrium which is disturbed only by an accvimulation 

 of stress, due perhaps to some unusual environmental condition, and 

 that when thus disturbed the plates fall into an entirel}^ different 

 plate pattern, much as patterns change in a kaleidoscope. The indi- 

 cations are, then, that in this genus species formation takes place 

 by changes which are in the nature of saltations, rather than by means 

 of fluctuating continuous variations. 



Major Skeletal Characters of Peridinidae 



Upon attempting to arrange the large number of species which 

 come under the definition of the genus, Peridinum, upon a systematic 

 basis useful for the taxonomist and expressive, so far as possible also, 

 of the natural affinities of the members of the genus, three prominent 

 skeletal characters present themselves for consideration. These are : 

 (1) the type of the antapical horns; (2) the direction of the dis- 



