420 University of California Publications in Zoology [Vol. 18 



direction along the horn in which the change in contour is slight and 

 gradual. 



In other genera, however, such as Pyrophacus and Gonyaulax, con- 

 ditions are present which suggest the pos.sible splitting of plates into 

 subequal portions, due to internal strains. In both of these genera, 

 but more especially in Pyropliacus, the number of apical plates varies 

 greatly among the several species of the given genus. In the apical 

 regions in Pyropliacus a cluster of as many as fourteen plates may be 

 found irregularly arranged and not to be clearly traced in rows. 



In this same connection it is also of interest to note that in differ- 

 ent genera it is different regions of the shell which are variable, sug- 

 gesting possibly a difference in the nature of the initial force influ- 

 encing the change as well as a difference in the location of the point 

 of yielding to these strains. Thus in Pyrophacus the yielding seems 

 to occur in the apical region, while in Peridinium the alterations seem 

 to involve a swelling of the circumference. This circumferential en- 

 largement involves a readjustment between previously existing apical 

 and cingular plates and the stretching apart of these two rows at 

 some place. This separation seems to occur on the dorsal side of the 

 epitheca rather than on the ventral side, and if the organism were 

 oriented horizontally this might be described as a dorsal humping of 

 the body with an anterior concavity in the hump. To meet this strain 

 and to cover the area which would otherwise be left vacant by the 

 stretching apart of the apical and precingular rows of plates on the 

 dorsal side, the dorsal epithecal accessory plates are formed. Once 

 having been established the strains seem to have been satisfied for 

 most members of the genus by the introduction of but three such 

 plates. 



We are also interested in a peculiar manifestation of the polarity 

 of these organisms. The hypotheca in these forms is consistently 

 more conservative than the epitheca in regard to the multiplication 

 of plates. The number of plates in the hypotheca is usually less than 

 the number in the epitheca and never exceeds this number. We do 

 not find in the genus immediately under consideration nor in related 

 genera the evidences of reconstruction or rearrangement of plates in 

 the hypotheca which we find in the epitheca. On the other hand there 

 are certain modifications of the hypotheca, but these are not of the 

 character to be caused by internal forces, but rather seem to be im- 

 pressed upon the organism by the environment. Thus the develop- 

 ment of hollow antapical horns is supposed to be due, partly at least, 



