Peridw liacese (5 1 



halves of the cell-wall are composed of definite series of plates 1 . There 

 are two transverse encircling series of plates in each half; that is, on 

 each side of the girdle. The apical part of the epivalve is composed 

 of the apical plates, either arranged as a cluster (Oxytoocum, Amphidoma) 

 or in a series grouped about an open apical pore (Ceratium and some species 

 of Peridinium). Behind these plates, and bordering the girdle, is the second 

 series, generally more numerous and of larger size, the precingular plates. 

 In a few genera a third series occurs between these two upon the dorsal side 

 and often asymmetrically displaced to the left ; these are the anterior inter- 

 calary plates, which never form a complete encircling series. There are two 

 or three of them in Peridinium, and one in Heterodinium. In the hypovalve 

 there is a series of postcingular plates immediately behind the girdle, usually 

 fewer than in the precingular series and sometimes of larger size. The 

 remaining part of the hypovalve, which is the posterior extremity of the cell, 

 consists of one or two antapical plates. A solitary posterior intercalary plate 

 may also occur. 



The series of plates in both the epivalve and the hypovalve may be 

 interrupted by a forward or backward extension of the longitudinal furrow 

 or ventral area. This interruption in the mid-ventral line forms a convenient 

 starting-point for numbering the plates of each series, the order being con- 

 tinued in the direction taken by the transverse flagellum ; that is, from 

 left to right. Kofoid's method of designating the various series is by using 

 acute accent marks from ' to "" for the four series, and a and p respectively 

 for the anterior and posterior intercalaries. Thus, the cell-wall of one of the 

 Peridiniacese, which contained the maximum number of plates, would consist 

 of: Apicals 1' 4'; anterior intercalaries l a 3 a ; precingulars 1" 7"; 

 postcingulars V" 5'"; posterior intercalary 1^; antapicals 1"" and 2"". 



So little is known of the plates which form the girdle, and of those of 

 the ventral area, that they have not been included in this general system 

 of nomenclature. There are six girdle plates in Gonyaidax and three in 

 Peridinium. 



Schiitt ('95) has shown that the plates can only be accurately determined 

 by following the sutures, and that the superficial ridges, etc., frequently offer 



1 The present author (Gr. S. W. '09) has observed a delicate tracery on the two valves of 

 Glenodinium uliginosum exactly resembling the suture-lines of certain of the smaller species of 

 Peridinium. Schilling ('91) also records such an instance. This fact, along with its thick wall 

 of three distinct parts and its method of cyst-formation, indicates that Glenodinium, although 

 intermediate between Gymnodinium and Peridinium, is much more closely allied to the latter 

 than the former and is rightly placed in the Peridiniaceae. Klebs has recently placed Glenodinium 

 in the Gymnodiniaceee, but on grounds which do not appear to be sufficient. Following up 

 Klebs' suggestion, Schilling ('Dinoflagellatae ' in Die Siissiv.-Jl. Deutschl. Osterr. u. der Schweiz, 

 Jena, 1913) has replaced the names Gymnodiniaceas and Peridiniaceae by ' Kyrtodiniacese ' and 

 ' Krossodiniaceaa ' respectively. 



