358 University of California Publications in Zoology. [Vol.4 



well-developed pores. They did not merge gradually into the 

 thin plates upon which they abutted but dropped off abruptly at 

 the suture lines. Their thickness was not less than five times 

 that of the adjacent new plates. The thin-walled plates of the 

 anterior schizont included the four apicals, l'-4' and precingulars 

 3" and 4". They were uniformly thin and hyaline and their 

 pores were scarcely visible. The plates of the apical series were 

 least regenerated, and were, indeed, barely formed at the distal 

 end of the apical horn. 



The condition of all three horns in this specimen was indica- 

 tive of their recent autotomy. In C. arcuatmn in normal condi- 

 tion (fig. 25) the antapicals are much longer than in this speci- 

 men and have pointed closed tips. The apical is also normally 

 of at least twice the length in this individual. In this specimen 

 all three horns are abnormally short and the antapicals are 

 abruptly truncated and open. The three horns are, moreover, 

 roughly still in the normal proportions of the horns in arcuatmn 

 to which species this individual appears to belong. The autotomy 

 of the two antapicals is here attended by a proportionate reduc- 

 tion in the length of the apical. 



The conditions here presented by this specimen unquestion- 

 ably indicate a process of exuviation in which a thecal wall of 

 senile character is in the process of being dropped off plate by 

 plate and replaced by a new wall of delicate texture. The or- 

 ganism does not abandon its old theca as do Glenodinium and 

 Gonyaulax, but drops it oft' piece-meal. Of the fifteen main 

 plates of the theca but four here remain of the old type. 



It may be significant in this specimen that all four of these 

 plates belong to the anterior segment. It is thus possible that 

 this exuviation attended schizogony and that the anterior seg- 

 ment shed its plates and regenerated new ones of a type similar 

 to those forming on the posterior segment as a result of the for- 

 mation after schizogony of new plates over the whole body, be- 

 neath the old on the anterior segment as well as over the posterior 

 segment. The formation of the new plates beneath the old on 

 the anterior moiety would result in the release and falling apart 

 of the superimposed old plates of the anterior segment. It seems 

 probable also that autotomy of the horns of a regulative or com- 



