440 University of California Publications in Zoology [Vol. 18 



plate lies. This pair may appear in one of two ways: either, appar- 

 ently as an increase in the number of apical plates from four to six 

 by the addition of two new plates arising at the dorsal corners of 

 apical plates 2' and 4'. These may have increased in size until they 

 reached the apex (as shown in fig. 10) and come to simulate apical 

 plates, though really of secondary origin. This arrangement of plates 

 is illustrated by very few species. If these accessory plates, however, 

 arose from the posterior corners of apical plate 3', their increase in 

 size may at first have produced the arrangement of plates given in 

 fig. 11, and later that of fig. 12. Examples of all of these patterns 

 have just been mentioned (see p. 410). At this stage, however, 

 a single unpaired plate may have made its appearance just posterior 

 to the junction of the two newly developed accessory plates shown in 

 fig. 12, i.e., at the anterior point of the mid-dorsal precingular plate 

 of this figure, or just anterior to the junction of the two accessory 

 plates at the posterior corner of the mid-dorsal apical plate (3'). 



Here, then, are two lines of development suggested for the intro- 

 duction of the accessory plates. The first, that sliown in fig. 10, seems 

 to have permitted no further progress, while the second, that illus- 

 trated in figs. 11 and 12, seems to have been that leading to such an 

 arrangement of plates as is shown in fig. 5, and found in a large num- 

 ber of species of this genus. This in turn by increasing growth of 

 the middle accessory plate probably led to the arrangement of plates 

 shown in fig. 6, also frequently found among species of Peridinium. 



The absence of a single unpaired accessory plate need not be re- 

 garded, however, as out of harmony with the occurrence of single 

 accessory plates described for such genera as Ceratocorys, Spiraulax, 

 and Heterodinium, since the accessory plates in these genera occur 

 asymmetrically and in Ceratocorys not at all in the same region as in 

 Peridinium. Moreover, the occurrence in Peridinium of two accessory 

 plates together, as in the first instance, may be but the complete sym- 

 metrical progression of the same sort of development which produced 

 but a single such plate on the right shoulder of Spiraulax or Hetero- 

 diiiium, and we may yet expect to find a similar asymmetrical Peri- 

 dinium caused by some aberration or partial inhibition in its normal 

 development, unless in fact one of these other genera may itself 

 represent this asymmetrical stage. 



It is doubtful, however, from which plate or plates these two acces- 

 sory plates may have come. The fact that in many other species the 

 mid-dorsal precingular plate, 4" seems to possess a certain plasticity 



