10 



STUDIES IN THE MORPHOLOGY, TAXONOMY, AND ECOLOGY OF THE PERIDINIALES 



The comparison of the morphological features of 

 the ventral areas of the forms in this report indicates 

 that Goniodoma is the most primitive genus investigat- 

 ed. This is shown by the simplicity of contours, size 

 equality, and small number of plates, viz., five, and by 

 the closed sulcal plate ring around the pore. At the 

 same time, the fact that the posterior sulcal plate is in 

 contact with the pore shows that Goniodoma does not 

 lie on the main line of descent but that it represents a 

 short, independent evolutionary branch near the base of 

 the general evolutionary path. Closely related to Goni - 

 odoma , but distinctly more advanced, and representing 

 a short, special side branch, is Ceratocorys , which has 

 the same number of sulcal plates. In this genus, how- 

 ever, the ventral area is narrowed, the sulcal ring is 

 broken, and the posterior sulcal plate is somewhat en- 

 larged. Still more advanced than Ceratocorys , and 

 closely related to each other, are Gonyaulax , Acantho - 

 gonyaulax , and Spiraulax, in which the ventral areas are 

 elongated and sigmoid, the posterior plate is greatly 

 expanded, and the number of plates has increased to six 

 or seven. Highly specialized and widely separated from 

 all the above genera and also from each other are Peri - 

 dinium and Ceratium . The ventral area of Peridinium 

 has a complex arrangement of intricate plates and lists, 

 whereas in Ceratium this structure is a wide, flat ex- 



panse of hyaline plates. 



The comparison of the girdle plates corroborates 

 the above conclusions, inasmuch as it separates the ge- 

 nera Peridinium and Ceratium from the others. At the 

 same time, it indicates that these two genera are mutu- 

 ally closer than either of them is to the remaining genera. 



The comparison of the total number of hypothecal 

 plates shows an exact parallelism with that of the gir- 

 dle plates; there are seven in Peridinium and Ceratium 

 and eight in the others, thus again bringing Peridinium 

 and Ceratium together. 



The patterns of the hypothecal plates separate 

 Goniodoma from Gonyaulax , Acanthogonyaulax . and 

 Spiraulax , showing, as is stressed above, that Gonio - 

 doma does not form a direct ancestral type. 



The comparison of the epithecal plates does not 

 contribute to this picture, but rather indicates an inde- 

 pendent structural differentiation. No two of the genera 

 have the same epithecal formula. 



Thus we are able to build up a plan of evolution and 

 relationships of the genera treated in this report, based 

 primarily on the ventral area and nicely corroborated 

 by the girdle and hypothecal tabulations (see fig. 2). As 

 more accurate knowledge is gained of the other genera 

 of the Peridiniales, it is hoped that these, too, will fit 

 into this general scheme. 



