(50 Peridiniex 



adopted. It is the simplest and the most easily applied, having a definite 

 morphological basis which admits of an accurate comparative treatment 

 of all the genera of the Peridiniacese. It recognizes that all the plates 

 are in transverse series, and that the girdle (or transverse furrow) rather 



Fig. 43. Gonyaulax spinifera (Clap. & Lach.) Diesing, to show the plates of the cell-wall. 

 A, dorsal view; B, ventral view; C, antapical view; D, apical view; 1 6, girdle series of 

 plates; 1' 3', apical series; 1" 6", precingular series; V" 6'", postcingular series; 

 IP, posterior intercalary; 1"", antapical plate; ant. pi., anterior plate of ventral area; 

 ant. sp., antapical spine; cl.pl., closing plate of apex; f.L, fission line along which the wall 

 parts in cell-division; fl. po., flagellar pore; int. pi., intermediate plates (usually four in 

 number, hidden in constricted region of ventral area) ; post, pi., posterior plate of ventral 

 area; v.a., ventral area (= longitudinal furrow) ; v. po., ventral pore, x 1000 (after Kofoid). 



than the equator should be used as a basis, since the plates are invariably 

 arranged with reference to the girdle, which is not necessarily equatorial in 

 position. 



The girdle (or cingulum) separates the epivalve from the hypovalve, 

 and in all the genera except Gienodinium and Ptychodiscus, these two 



