444 University of California Publications in Zoology [Vol. 18 



and that later a third plate is added between them, making the num- 

 ber three characterstie of the greater part of the genus. Upon the 

 changes of articulation of this middle accessory plate, 2a, are rung 

 four changes of plate pattern, depending largely upon the degree of 

 growth and symmetry of growth of this plate. Under the symmetrical 

 development of the plate the patterns of figs. 5 and 6 only are pro- 

 duced. By the unequal development of what are apparently a pair 

 of centers of change, either pattern of figs. 7 or 8 is produced. These 

 latter patterns appear under conditions of unusual modifications of 

 the environment, and these changes in pattern involve the articulations 

 of plates in the vicinity of the anterior corners of the mid-dorsal pre- 

 cingular plate, 4". 



The presence, however, of the two skew patterns, the reverse of 

 each other, shows that a double center of variation or a pair of vari- 

 able areas are involved, and that, whereas in the majority of cases 

 the development in these two centers proceeds at a uniform rate, 

 there are cases in which for some reason this balance is upset, one 

 center developing more rapidly than that on the opposite side, pro- 

 ducing as a result the asymmetrical dorsal plate patterns of figs. 7 

 and 8. 



Paired Areas of Change of Plate Patterns 



The fact is striking that there are on the ventral and also on the 

 dorsal surfaces of the skeleton of the epitheca paired areas of change 

 of plate pattern which seem to be due to some internal stresses which 

 at first seem to have made for the production of additional plates in 

 the history of the forerunners or early members of the genus and 

 which later seem to have manifested themselves in modifying the 

 arrangement of a constant number of plates. 



The pair of variable ventral areas present coincident modifications 

 in two subgenera of the genus, Paraperidinium and Orthopcridinium, 

 but may vary independently of each other, as is shown in Metaperi 

 dinium. Strangely enough, a fourth group to be represented by the 

 reverse asymmetrical pattern from that of Metaperidinium seems not 

 to be found in our present fauna. Its absence, however, is to be re- 

 garded as of only negative significance*. 



Similarly, the variable dorsal areas usually develop coincidently, 

 but under certain circumstances behave individually. It may be 



