FAMILY PERIDINIACEAE 



13 



the rabbeting membrane is apparently of the same 

 width in specimens with and without intercalary zones, 

 there must be some intercalated material at the inter- 

 nal suture as well as at the external suture, as shown in 

 figure 3B. If this be true, then there is an internal as 

 well as an external intercalary zone with the internal 

 suture bisecting it approximately in the middle. 



Lists . Although never developed to the extreme de- 

 gree that they are in some members of the Dinophysi- 

 ales, nevertheless, lists form a prominent part of the 

 thecal morphology of Peridinium . The girdle lists are 

 always well developed and the posterior girdle list is 

 usually confluent with the sulcal lists. The ventral area 

 has a complex system of lists. This area is usually en- 

 closed by the right, posterior, and left sulcal lists, and 

 internal to these there may be right, posterior, and left 

 accessory sulcal lists. These will be described under 

 each species. The apex is another region of complexi- 

 ty as regards lists. It is encircled by a list made up of 

 parts of three lists attached to the apical plates. These 

 lists always extend down the ventral side of the body as 

 far as the posterior end of the ventral apical platelet, 

 but may extend down the ventral and lateral sides of the 

 body part way or entirely to the girdle. These will be 

 described in detail in the individual species descriptions. 

 No prominent body lists are developed except along 

 plate sutures. The transverse ridges across the girdle 

 sometimes, however, take on the character of lists. 



Spines . Antapical spines are developed in some 

 species (e.g. P. pallidum ) in place of antapical horns. 

 No other spines occur. The "apical spines" and the 

 "antapical spines," internal to the antapical horns 

 sometimes indicated in published figures, are not spines 

 at all but are optical effects produced by the curvature 

 of lists. 



Historical Review 



Peridinium is the largest and probably the most 

 widespread genus of the Peridineae. More than two hun- 

 dred species have been described. Thirty-eight of these 

 are fresh-water forms, the rest are marine. 



There are many morphological characters in addi- 

 tion to the general body shape which can be used in de- 

 fining the species. Unfortunately, however, these char- 

 acters are variable. Because of the pronounced intra- 

 and interspecific plasticity, the genus is the most diffi- 

 cult in the Peridineae from the standpoint of thesystem- 

 atist. So far no one has had the courage to attempt a 

 generic monograph, a fact which is fortunate since it 

 would, necessarily, have been only an abortive attempt 

 because of our ignorance in regard to the pertinent data. 

 Such a treatment can be made only after we have ac- 

 quired a thorough knowledge of the thecal morphology of 

 each of the species. 



Furthermore, the generic concept is one of the 

 vaguest in the Peridineae. It has been very changeable, 

 being sometimes narrow, sometimes broad. For exam- 

 ple, Lindemann's concept (1928) of the genus included 

 four described genera besides Peridinium of previous 

 authors. Generally speaking, the confusion in the genus 

 progressed as the number of described forms increased. 

 It is fair to predict, however, that, when all the mem- 

 bers of this vast genus have been thoroughly analyzed, a 

 satisfactory classification will be evolved in spite of the 

 variation in some features which now baffle our attempts. 



The first grouping of the species of Peridinium was 



made by Gran (1902). He divided the genus into two 

 subgenera: Protoperidinium and Euperidinium . Proto - 

 peridinium was adopted by Gran fromBergh (1881), who 

 applied this name to a group of species to which he as- 

 signed generic value. The name Euperidinium , on the 

 other hand, was introduced by Gran. Protoperidinium . 

 according to Gran, is characterized by solid antapical 

 spines and by a right-handed girdle, i.e., on the ventral 

 side of the body the right (distal) end of the girdle is 

 located anteriorly to the left (proximal) end. Euperi - 

 dinium is characterized by hollow antapical horns and 

 by a left-handed girdle, i.e., the left end of the girdle is 

 displaced anteriorly. Gran listed the following species: 

 Protoperidinium : P. p ellucidum Bergh s.L, P. ovatum 

 Pouchet, P. decipiens Jorg., P. steinii Jorg., P. globulus 

 Stein, Euperidinium : P. conicus Gran, P. pentagonum 

 Gran, P. divergens Ehr. s.s., P. depressum Bail. s.L 



This classification was satisfactory for the few 

 species that were known at that time and was accepted 

 by many authors. Later investigators, however, re- 

 vealed forms which could not be classified on the basis 

 of these criteria. For instance, forms were found with 

 a left-handed girdle, but with solid antapical horns. 

 Furthermore, forms with no definite displacement of 

 the girdle in either direction were discovered. 



Paulsen (1908) used Gran's classification in the 

 treatment of thirty-nine species of Peridinium although 

 he was compelled to acknowledge exceptions to the sys- 

 tem. Thus, in Protoperidinium he included two species, 

 P. findlandicum Paulsen and P. granii Ostenfeld, which 

 have hollow horns. Those forms in which there is no 

 displacement of the girdle he placed in Euperidinium . 

 The description of this subgenus was further qualified 

 by the statement that there are usually hollow horns. 

 Paulsen was the first to show discrepancies in theGran- 

 ian classification. 



Broch (1910) noted the inadequacy of the classifica- 

 tion based on the displacement of the girdle and the struc- 

 ture of the antapical horns. He also pointed out that the 

 arrangement of the plates of the theca afforded a ready 

 diagnostic means. Although he grouped his ten species 

 from Val di Bora under the subgenera Protoperidinium 

 and Euperidinium , he carefully described under each 

 species the conformation of the first apical plate and 

 figured the plate pattern for each form. 



Finally, Jorgensen (1913) made a comprehensive 

 reclassification of the genus on the basis of the plate 

 pattern, using the number of plates that border the first 

 apical as a primary character in the formation of sub- 

 genera and the pattern of the dorsal plates as the cri- 

 terion for the division of the subgenera into sections. 



Jorgensen divided the genus Peridinium into two 

 subgenera, Orthoperidinium and Metaperidinium . and 

 discarded the subgeneric names used by Gran. Ortho - 

 peridinium is characterized by a first apical which 

 touches only four of the major plates of the epitheca; the 

 second apical and first precingular are on the left side, 

 the fourth apical and seventh precingular on the right 

 side (fig. 4A). Metaperidinium is characterized by a 

 first apical plate which borders five or six of the prin- 

 cipal plates of the epitheca; the second apical and the 

 first and second precingulars are on the left side, the 

 fourth apical and the seventh, or the sixth and seventh, 

 precingular on the right side (fig. 4B, C). 



Orthoperidinium was divided into three sections, 

 principally on the basis of the dorsal epithecal plates, 

 as follows: 



