1918] Barroivs: Skeletal Variations in the Genus Peridinium. 459 



2. In the classification of species of Peridimimi skeletal characters, 

 particularly the relationships of the fundamental skeletal elements, 

 the plates, are of much more real value than the type of antapical 



3. The size which these plates in Peridinidae attain is limited, and 

 is apparently correlated somewhat with the degree of curvature .of 

 the shell. The dimension of the plate is usually longer in the direction 

 of slight curvature than it is in the direction of great curvature. In 

 certain regions, e.g., the middle of the dorsal surface or of the apical 

 horns in Peridinium, the expansion of a given plate seems to have 

 become accomodated to the expansion by an extension within limits, 

 in the direction of expansion, of the plates overlying the expanded 

 portion of the body, rather than by a shifting of the surrounding 

 plates to distribute the strain. The direction of expansion, however, 

 is usually not in the form of a curve. 



4. There is evidence to suggest that new plates are added to fill 

 gaps occurring when the three plates meeting at a given point are 

 separated by some internal pressure, rather than that new plates are 

 split off from previously existing plates. 



5. At a certain stage among the genera in the part of the family 

 in which Peridinium belongs, the number of plates in the precingular 

 row increases so that these plates can no longer reach the apical row 

 and an accessory row of plates is interpolated on the dorsal side 

 between the two previously existing rows. 



The progress of this increase in the number of the plates in the 

 precingular row may be traced from such genera as Heterodinium, 

 Spiraulax, and Gonyaulax which have six precingular plates (the 

 plate of the right hand end of this row, 6", frequently being of small 

 size), through the several stages of development pointed out in the 

 three subgenera of Peridinium to such an exceptional form as that 

 figured by Faure-Fremiet (1908, p. 227, fig. 13) for P. minutum 

 Kofoid var. tatihoiisensis in which there is apparent an eighth pre- 

 cingular plate at the right hand end of the row. 



6. In Peridinium the first plates to appear in this accessory row 

 come in as a pair of plates. It is not until a later stage that a third 

 accessory plate makes its appearance between the original pair. Sub- 

 sequent changes in the arrangement of the dorsal plates occur over 

 the spots on the body of the organism at which the two accessory 

 plates appeared. The addition originally of but a single accessory 

 plate is unknown. 



7. On the ventral surface of the organism additional plates added 



