86 MEMOIRS OF THE UNIVERSITY OF CALIFORNIA 



girdle may form a closed circle (figs. X, 1, 2, 8, 17) or its distal end may become 

 displaced iiosteriorly to any distance less than 0.2 of the total length of the 

 body (fig. Y). The extreme of this condition is found in G. ruhrum (fig. C, 5), 

 which closely approaches the next genus, CochlocUnium (fig. C, 6), in its tj^^e 

 of girdle arrangement. 



The advance from the genus Gymnodiniiim to Gyrodinium is but slight in 

 the species at the adjacent extremes of both genera (figs. C, 5, 6) . Within the 

 latter genus, however, the progress made in increasing development of the girdle 

 and sulcus is considerable. Here the ends of the girdle are displaced about 

 0.2 of the total length of the body at the lower end of the scale (fig. C, 6) . From 

 this point upward an increasing progressive displacement of the ends of the 

 girdle is found (fig. CC) which reaches its greatest extent in 6^. cornutum (fig. 

 C, 7). Avhere it is slightly more than 0.5 of the total length of the body. The 

 girdle also increases in length so that more than one complete turn of the body 

 is made (fig. C, 7). This results in a torsion of the body and also of the sulcus 

 which is carried about with it. In this genus the torsion never becomes as 

 great as 1.5 turns of the body (fig. CC). 



In the torsion of the body the next genus, Cochlodinium, exhiljits the maxi- 

 mum of specialization found in the Gymnodinioidae. The amount of torsion 

 varies from 1.5 in those species near the border line separating this genus from 

 Gyrodininm (fig. C, 8) to four complete turns of the body in the most highly 

 differentiated species (fig. C, 10). Between these two extremes all gradations 

 in the amount of torsion may be found (figs. GG, HH). Torsion of the sulcus 

 also reaches its maximum in this genus, always having approxunately one less 

 turn than that of the girdle because of their morphological relations. 



The lines of separation of Amphidininni, Giimnodinium, Gyrodinium, and 

 CocModininm must of necessity be somewhat arbitrary, since the great increase 

 in the number of species results in an increase also in the number of inter- 

 mediate forms linking together the extremes of each genus. The gap between 

 nemidinium and the other members of the family is much harder to bridge. 

 It is a genus of little known, fresh-water forms of which two species only have 

 been described. 



The remaining genus of the family, Polyhrikos, is a colonial Gymnodinium 

 which has secondarily developed nematocysts, and may be considered an off- 

 shoot of that genus. Temporary colonial chain formation is frequently seen in 

 Gymnodinium and Cochlodinium as well as in the thecate forms, Ceratium, 

 Peridinium, and Gonyaulajc. The colonial organization of Polyhrikos, how- 

 ever, seems to have acquired permanency. It shares with the genus Nemato- 

 dinium the distinction of being the only genera in the Dinoflagellata possessing 

 nematocysts. 



The five genera of the family Pouchetiidae present another clear instance 

 of progressive orthogenetic diiferentiation culminating in the most complex 

 types of the GjTnnodinioidae, even of the Dinoflagellata, with respect to the 



