1918] Barroivs: Skeletal Variations in the Genus Peridinium 413 



occurring- forms having plate pattern of figure 1 he gave the group 

 name, Paraperidinium, but in his later treatment he merged this 

 group under Metaperidinium. 



Joergensen also recognized that in both of those groups having 

 plate patterns 3 and 4a and named by him Metaperidinium and 

 Orihoperidinium, respectively, three different dorsal patterns of the 

 plates of the epitheca also occurred. He accordingly redivided each 

 primary subdivision of the genus on the basis of the dorsal pattern, 

 and established under Metaperidinium a seventh place for the few 

 erratic species having the pattern of Paraperidinium for the ventral 

 epithecal plates. Though certain corrections and amplifications may 

 be made in this system as given by Joergensen, its formulation was a 

 great step in advance toward a fuller and more correct understand- 

 ing of the organization of the genus than had previously existed. 



Pavillard (1916) summarizes Joergensen 's system as follows: 



Orthoperidinium Metapendimum 



Sec. I. Tabulata Sect. IV. Pyriformia 



e.g., P. tabulatum e.g., P. steiiiii 



Sect. II. Conica Sect. V. Paraperidinium 



e.g., P. conicum e.g., P. pallidum 



Sect. III. Oceanica Sect. VI. Eumilia 



e.g., P. depressum e.g., P. ovatum 



Sect. V. Divergens 



e.g., P. crassipes 



Pavillard (1916) criticizes Joergensen 's system on the basis that 

 there is no real relation between those members of the group Oriho- 

 peridinium and Metaperidinium, which have the same dorsal pattern, 

 nor between the species of these two subgenera differentiated by hav- 

 ing different dorsal patterns and connected on the basis of similar 

 ventral patterns, as members of linear series. Pavillard accordingly 

 proposed another grouping of these seven subdivisions, accepting in 

 the main Joergensen 's definitions based upon plate patterns. Pavil- 

 lard also finds a few species which cannot be included in any of these 

 seven subdivisions which he appends as "Incertae Sedis. " P. curvi- 

 pes, P. mite, which are said to intergrade, and P. rectum, said to be 

 unstable. 



This historical resume of the various bases for classification in this 

 genus has been introduced to show the recent trend toward recogni- 

 tion of the importance as well as the convenience of the pattern of 

 the plates in the organization of the genus. 



