1918] Barrows: Skeletal Variations in the Genus Perkliniiim 423 



certain species of Ceratium and Gonyaulax strengthens this conclusion. 

 Further, the fact that plates rarely exceed a certain maximum size, 

 and then only in the case of fairly flat portions of the shell, suggests 

 the existence of some factor either within the organism itself or within 

 the material of the shell M^hich makes for a fragmentation of the shell 

 into plates of a restricted size. 



Regions of Skeletal Variation in Peridinium 



AVe are now, perhaps, in position to appl}^ these observations to 

 members of the genus, Peridinium. Aside from variations in the 

 plates of the ventral area which are imperfectly known because of 

 the great difficulty in analyzing them there are no variations in plate 

 articulations of the hypotheca within this genus. The principal 

 morphologic variations concern the development of antapical horns, 

 and as has been pointed out above, these cannot be regarded as of pre- 

 dominating significance. There is, however, variation not only in the 

 number of plates of the epitheca but also much variation in the 

 arrangement of the number of epithecal plates (14) which is of most 

 frequent occurrence. These variations are found to occur in two 

 general regions ; one, on the ventral side of the epitheca on either hand 

 of the rhomboid plate (l^j aiid the other on the dorsal side between 

 the articulation of the partial row of three (in typical cases) acces- 

 sor}' plates, presumably of recent phylogenetic origin, and the mid- 

 dorsal precingular plate (4"). There is, however, a considerable 

 number of species in which the partial row of accessory dorsal plates 

 contains only two plates instead of three, the greatest number devel- 

 oped in this row in this genus. These species, as recorded, however, 

 all have the same ventral plate patterns. The variations in the 

 vicinity of the mid-dorsal precingular plate (4'') are such as might 

 be expected to accommodate the smaller number of accessory plates 

 present. 



The regions in which these articulations change will be seen from 

 the accompan3dng diagram (fig. 19) of an apical view of Peridinium 

 divergcns Ehrbg., showang all of the epithecal plates in this, a typical 

 species of this genus. 



It is to be noted that there are only four points of the intersection 

 of sutures between plates at which changes in articulations occur. 

 These four points are paired symmetrically with respect to the sagittal 

 plane, and changes of articulation seem usually, though not always, 



