1918] Barrows: Skeletal Yariations in the Genus Peridinium 427 



ization and of evolution in this group— is by the formation of new 

 plates in the row already represented. This method is known to 

 have been carried to considerable development, for instance in such 

 forms as Pyrophacus, in which the plate formula is 4 (to 12) '-0^-9 

 (to 12)"-s-9 (to 12)'"-0'^-3 (to 5)"". Another method for in- 

 creasing the number of plates which must involve a greater readjust- 

 ment of the skeleton is by the addition of what is to become either 

 the anterior or posterior accessory row of plates. Such plates usually 

 make their appearance on the dorsal side of either the epitheca or of 

 the hypotheca. In other genera we do not find this method resorted 

 to until the precingular and postcingular rows have broken up into 

 at least five plates each, and in many genera not until six or seven 

 plates have been formed in these rows. It would therefore seem that 

 the increase of the number of plates to a certain point in the cingular 

 rows is a phenomenon generally prior to occurrence to increase by 

 the addition of an accessory partially complete row, and that increase 

 of plates in an existing row is a simpler and less highly specialized 

 process than the addition of a new row of plates. It seems reasonable 

 to suppose, therefore, that the changes in the ventral plate pattern 

 in the genus, Peridinium, are of more fundamental significance than 

 changes in the dorsal pattern. 



Moreover, in certain genera closely related to Peridinium there is 

 a suggestion that the increase in number of precingular plates comes 

 from the addition of a new plate at one of the ends of this row on 

 the ventral side near the longitudinal groove. Thus in Gonyaulax, 

 plate 6'^ is very small. A plate introduced at the end of the pre- 

 cingular row of a form ancestral to the genus, Peridinium, with its 

 seven precingular plates will probably be small in size in the early 

 stages after its appearance, but as this new plate increases in size to 

 match approximately the size of the other plates of the row it will 

 more and more nearly approach plate 4' of the preexisting apical row. 

 Thus in the early stages of the introduction of such plates at either 

 end of the ventral row the ventral plate pattern on the side of the 

 rhomboid plate would be represented by the pattern shown in fig. 1. 

 A later stage representing the full growth in size of one of these 

 plates would be represented in fig. 3, and if growth should take place 

 to the same degree on both sides of the rhomboid plate a pattern sim- 

 ilar to that of fig. 4a would result. Of the three ventral patterns 

 known, therefore, that of fig. 1 seems to be the more primitive and 

 that of fig. 4fl the more highly specialized. 



