454 University of California Publications in Zoology [Vol. 18 



There is therefore effected a sort of limited kaleidoscopic move- 

 ment of the plates upon each other in response to internal stresses 

 with sufficient remodelling of the margins of the plates to establish 

 contact sutures of definite and considerable length. This all goes to 

 support the view that the plates of one pattern are in a state of 

 equilibrium which requires a certain amount of accumulated strain 

 before transformation into the alternate pattern can occur; that each 

 pattern represents a state of approximate equilibrium of the skeletal 

 elements out of which the plates may be forced only by the accumula- 

 tion of an unusual stress sufficient to throw the plates over into an- 

 other arrangement of equilibrium in the alternate pattern. 



The equilibrium of each pattern seems usually to be characteristic 

 and constant for each species. The abruptness of the transition from 

 one pattern to another is illustrated in the case of P. divergens just 

 described. This transition, however, is not accompanied by other 

 morphological changes sufficient to warrant the description of a new 

 species. As a rule it may be supposed that changes which would 

 throw the plate patterns from one relation of equilibrium into another 

 would also be accompanied by such other structural changes as to 

 compel the consideration of the new form as a species. 



Particularly with the pair of dorsal patterns in P. divergens 

 Ehrbg. is it to be seen from the curves charted that in the groups of 

 specimens from each locality the trend of variation of the pattern 

 on both sides of the organism is much the same in each group, and 

 the mode for the length of both the right hand and left hand critical 

 sutures is about the same. This is in accord with the general observa- 

 tion of the dorsal symmetry of pattern in this genus, to which there 

 are but few exceptions, some of these occurring under conditions of 

 an unusual environment which may be presumed to have interfered 

 with the morphological equilibrium of a species. 



The ventral patterns are, however, not so closely coupled as was 

 noted in defining the subgenera of this genus, Paraperidinium, Meta- 

 peridi7iium (dextrad), and Orthoperidinium, on the basis of ventral 

 plate pattern. 



It is presumed that at least one of the specimens figured by ]\Iangin 

 showing the Paraperidinium pattern does not differ otherwise from 

 the average run of specimens of P. divergens. The other specimen 

 differs somewhat in surface markings, but this may be merely an age 

 difference. These specimens are, therefore, regarded as specimens of 

 the species, P. divergens, which show an unusual form of plate pattern. 



