446 University of California Pithlications in Zoology [Vol. 18 



COMPLKTENESS OF THE SeRIES OF VARIATIONS 



Geometrical Patterns Represented. — Another significant fact is 

 that, given the constant number of plates on the ventral surface of the 

 epitheca, all possible geometric combinations of these plates are repre- 

 sented among the species of this genus, except for the unknown group 

 which should have the asymmetrical pattern, the reverse of that of 

 Metaperidinium, and which we may call Metaperidinium (sinistrad). 



Similarly, on the dorsal side, given the constant number of plates 

 after the full development of three accessory plates, all geometric 

 combinations possible about the pair of dorsal regions of variability 

 are known. 



This complete series of combinations makes it possible to suggest 

 the manner by which these changes in plate pattern occurred and 

 confirms our confidence in the hypothesis suggested for the sequence of 

 these changes. The completeness of this series of combinations also 

 suggests the great individuality of each region of variability as a 

 character capable of variation independent, at times at least, of all 

 of the other similar characters of change ; and again, the great extent 

 to which this group of organisms seems to have expanded, occupying 

 all avenues of variation possible with a given structure. 



Completeness of Combinations of Patterns. — Dealing first with the 

 symmetrical patterns, both ventral and dorsal, we find that species 

 are known representing combinations of the symmetrical ventral pat- 

 terns of both of the subgenera, Paraperidinium and Orthoperidinium, 

 with both of the symmetrical dorsal patterns as represented in figs. 5 

 and 6. The patterns of the subgenus, Metaperidimum, also are known 

 in several species in combination with both of the symmetrical dorsal 

 patterns. 



It appears, therefore, that all of the combinations possible, six 

 in number, between the two symmetrical dorsal plate patterns and 

 the three known ventral patterns are represented more or less abun- 

 dantly in the peridinian fauna. This circumstance again confirms 

 the independence of the characters represented by the different pat- 

 terns and the wide range of the trials bj^ which nature seems to have 

 coupled these characters of plate patterns together, and also the success 

 which combinations of plate arrangement according to the three ven- 

 tral plate patterns, two of which are symmetrical, with the symmetri- 

 dorsal pattern. 



