Peridiniese 



no clue to the sutures. This has been still further emphasized by Kofoid 

 ('07 c), who states that the ' actual separation of the plates in situ is the only 

 safe guide to an analysis of the thecal wall.' 



The plates are often covered with minute spines, or with a close network 

 of ridges giving them an areolated appearance. Both pores and poroids 

 occur on the plates, only the former being real perforations through the wall. 

 The wing-like expansions which occur in some of the marine forms are 

 supported by variously arranged radial and transverse horns and ribs. These 

 expansions do not act merely as parachute-like structures : their disposition 

 is such that the sinking organism always assumes a definite position. The 

 expansions of Ceratocorys, Phalocroma, Ornithocercus, etc., and the apical 



Fig. 44. Peridiniuitt Willei Huitf.-Kaas, to show the plates composing the wall. A, ventral 



view; B, dorsal view; C, apical view; D, antapical view. 1' 4', apical plates; 1" 7", 



precingular plates; 1" 3 a , anterior intercalary plates; 1'" 5'", postcingular plates; 



1"" and 2"", antapical plates. The sculpture of the plates is not shown in the figures. 

 x500. 



and antapical horns of Ceratium, are always asymmetrically disposed. In 

 some species Kofoid ('10) has demonstrated that in the passive sinking of the 

 organisms, this fundamental asymmetry of horns and expansions causes 

 the body of the cell to turn on to its ventral face with the maximum 

 exposure of body and horns to the direction of sinking, thus delaying the 

 descent. 



The sutures of the plates are sometimes scarcely visible, as in Peridinium 

 berolinense Lemm., but more often the lines of junction are marked by 

 intercalary bands, which not infrequently possess prominent cross-striations 

 (figs. 44 and 45). These attain their greatest development in Peridinium 



