1918] Barrows: Skeletal Vanations in the Gcuus Peridiuium 461 



over modifications of the dorsal plate pattern. The absence from 

 present authenticated records of an asymmetrical ventral plate pattern, 

 the reverse of the pattern of the subgenus, Metaperidinium, can only 

 be given negative significance. 



13. Since the end plates in Ortlioperidinium are much larger than 

 in Paraperidinium, the former subdivision of the genus is supposed 

 to represent a more highly specialized condition in respect to plate 

 growth than that of Paraperidinium. Metaperidinium by reason of 

 the lagging of the growth of the plate at the left end of the precingular 

 row represents an intermediate stage of specialization. 



As might be expected in the more highly specialized portion of 

 the genus, there are, to judge from published records and figures, 

 more species in the subgenus, OrtJioperidinium, than in both of the 

 two other subgenera combined, indicating the prolific diversity of form 

 in highly specialized stages. 



14. Over half of the sixteen possible combinations between all 

 the dorsal and ventral plate patterns are known, including: all four 

 of the combinations between the symmetrical ventral patterns, Para- 

 peridiniimi and Ortlioperidinium, and the two symmetrical dorsal 

 patterns known; also combinations between one of the two asymme- 

 trical ventral patterns, Metaperidinium, and both of the symmetrical 

 as well as both of the asymmetrical dorsal patterns : and the combina- 

 tion of the ventral plate pattern of Ortlioperidinium with the right 

 oblique (asymmetrical) dorsal pattern, reported for at least one 

 species. One asymmetrical ventral pattern is wholly unknown and 

 combinations of the asymmetrical dorsal pattern with Paraperidinium 

 and Ortlioperidinium are also unknown. 



15. In that portion of the genus in which there are only two acces- 

 sory dorsal plates, whether symmetrical or of unequal size, the dorsal 

 patterns thus formed are known in combination only with the sym- 

 metrical ventral pattern of Ortlioperidinium. No instance is known 

 of the occurrence of but a single dorsal accessory plate in this genus. 



Of the species on record, many of those having but two accessory 

 plates are fresh water forms and most of the fresh water forms have 

 only two accessory plates. This primitive character for this genus 

 is, however, coupled with the presence of the specialized ventral plate 

 pattern of the subgenus, Ortlioperidinium. Such an unexpected com- 

 bination is perhaps to be explained on the hypothesis that the develop- 

 ment of accessory plates is of secondary importance to the rearrange- 

 ment of the plates developing about the rhomboid plate after the addi- 



