1918] Barrows: Skeletal Variations in the Genus Peridinium 445 



noted that whereas the asymmetrical ventral pattern is known in a 

 widely distributed portion of the genus represented in all seas, the 

 asymmetrical dorsal patterns are found usually, except for such a 

 form as P. steifiii Joerg., in specimens which have come from a region 

 of more or less modified environment from that of the high seas or 

 open coast, and also that the dorsal asymmetry, among species re- 

 ported, seems to be the more often produced by growth upon the left 

 side of the middle accessory plate than upon the right side. As evi- 

 denced in Metaperidinium, it is on the right side, however, that devel- 

 opment seems to proceed the more rapidly, and forms having a devel- 

 opment of the newly acquired precingular plate, which is more rapid 

 on the left side than on the right are unknown. 



Here there are apparently four potentially independent charac- 

 ters, which, however, more often than not, behave in dorsal and ven- 

 tral pairs. 



A second important consideration is that there is apparently no 

 particular connection between the dorsal and ventral pairs and, as 

 will be shown later, a full set of combinations is known between both 

 symmetrical patterns of the ventral side and the symmetrical patterns 

 of the dorsal side, and a number of the possible combinations of these 

 patterns with the asymmetrical patterns of both ventral and dorsal 

 sides are also known. 



It seems not impossible also that the character of a pair of horns 

 on the hypotheca, which may or may not be developed to approxi- 

 mately the same degree, may be correlated with the influence making 

 for the frequent pairing of the right and left characters of plate vari- 

 ation. Also, the initial occurrence of a pair of dorsal anterior acces- 

 sory plates sjrmmetrically placed, instead of a single accessory plate 

 upon perhaps the left shoulder, as in Heterodinium, is of particular 

 significance in connection with the occurrence of these other features 

 in pairs by emphasizing the fundamental and natural condition of 

 bilateral symmetry in this genus. If this be related here as elsewhere 

 to a method of rectilinear locomotion, it is suggestive of the super- 

 sedence of a structural response to locomotion of this sort over the 

 structural response suggested in the torsion of the shell by the addi- 

 tion of but a single accessory plate in other genera as a concomitant of 

 the spiral method of locomotion and the oblique strains set up by this 

 method. 



