452 University of California Publications in Zoology [Vol. 18 



places where changes of plate pattern seem to have occurred in the 

 formation of other species of the genus, but the value of the critical 

 suture in each case which must be obliterated in the formation of a 

 new pattern never quite approaches zero. 



On the left hand side of the ventral surface of the epitheca the 

 critical suture is that between plates 1' and 2". In a change to the 

 Orthoperidinium pattern this suture would be obliterated by the meet- 

 ing of plates 2' and V, which in P. divcrgens are separated. How- 

 ever, in P. divergens, there is never any suggestion of such an approxi- 

 mation of plates 2' and 1" and the suture between plates 1' and 2" 

 is always of considerable length. 



On the dorsal surface of the epitheca, the plate patterns at the 

 posterior corners of the middle accessory plate usually are sjTnmetri- 

 cal in this genus. In P. divergens, although varying through a wide 

 range of length, the critical sutures between plates la and 4" and 

 plates Sa and 4", which must be obliterated in the transposition to 

 the alternate patterns for these regions, never fall below .04 of the 

 length of the transdiameter. In the measurement of Broch's figures 

 these critical sutures are also found to preserve their definite and 

 considerable value. 



Here there are over 160 measured cases in which the length of the 

 critical suture in variable regions of the shell of specimens of P. diver- 

 gens, including the alternate unusual pattern in one of the variable 

 regions for two of the instances, does not approach zero in value, but 

 maintains a definite and relatively considerable length. This is main- 

 tained in spite of the fact that other species of this genus are funda- 

 mentally characterized by having a different plate pattern in these 

 critical regions. 



Now the significance of this fact seems to be this. Species in this 

 genus are evidently formed by the rearrangement of a constant num- 

 ber of plates in a limited number of possible geometric patterns. 

 There are in this genus only four regions where such adjustments 

 occur. At each region only two different patterns are possible, one 

 the alternate of the other. One pattern is changed into the other by 

 the expansion of one or both of a pair of diagonally opposite plates, 

 causing the separation of another pair of plates previously in con- 

 tact and the obliteration of one suture with the establishment of a 

 new suture. 



In specimens of P. divergens examined, the patterns of three of 

 the four regions of possible variation seem to be stable. The pattern 



