428 University of California PuhlicafAons in Zoology [Vol. 18 



There seems, then, to be a broad and sound basis for the division 

 of the genus into the two groups, Orthoperidinium and Metaperi- 

 diniuni, which Joergensen has defined, and quite as good ground for 

 giving his group Paraperidinium correlative standing. We would, 

 therefore, suggest that these names be preserved in common usage 

 for these three subgenera of the genus, Peridinium.. 



Subdivisions of the Genus, Peridinium 



It is possible, therefore, to divide the whole genus, Peridinium, 

 into three subgenera upon a basis which is not only of practical conven- 

 ience in diagnosis but which also has a natural significance. It might 

 be possible in some or all of these subdivisions to carry the classifica- 

 tion according to this system even farther on the basis of the com- 

 binations which a given plate pattern might form with the known 

 dorsal patterns. Certainly the characters next in value after the dorsal 

 pattern would be the characters of type of horn, general shape, surface 

 markings, etc. 



Of the four dorsal patterns figured, only the first two (figs. 5 and 

 6) occur extensively. Combinations are known between each of these 

 and each of the three known ventral plate patterns. The two remain- 

 ing dorsal plate patterns as shown in figs, 7 and 8, are known to be 

 combined with one of more of the three ventral patterns. 



Upon our interpretation of these ventral plate patterns and the 

 significance of the series, the group to which Joergensen has given the 

 name, Paraperidimum, becomes the first one to be considered, as prob- 

 ably representing the most primitive condition in which the plates 1" 

 and 7" at either end of the precingular series are of relativelj^ small 

 size, presumably having been recently introduced, Joergensen 's 

 group, Metaperidinium, represents an intermediate asj^mmetrical 

 stage. To distinguish this form from the possible inverse asymmetri- 

 cal pattern of fig, 2, which is unknown in reality, it may be called 

 Metaperidinium (dextrad), and the unknown form, Metaperidinium 

 sinistrad. 



Joergensen 's subgenus, Orthoperidinium, becomes, then, the most 

 highly specialized subdivision of the genus, 



Paraperidinium. — Taking up first the known forms presenting 

 combinations of the plate pattern of Paraperidinium with the sym- 

 metrical dorsal plate pattern of fig, 5 we may name the following 



