1918] Barroivs: Skeletal Variations in the Genus Peridinium 433 



Beside the species just enumerated as having dorsal plate pat- 

 terns of figs. 10, 11, and 12, which are fairly symmetrical, there are 

 at least three species in which the two dorsal accessory plates are dis- 

 posed of in a very asymmetrical pattern: 



P. excentricum Pauls. P. perrieri Faure-Fremiet 



(Pavillard, 1916, p. 31, fig. 4) (Faure-Fremiet, 1908, p, 228, 



P. paulseni Mangin fig. 14) 



(Mangin, 1912, p. 226, fig. 12) 



A single report is extant of a species of Peridinium without any 

 anterior dorsal accessory plates. This is P. umhonatum var. elpati- 

 ewskyi Ost. (Ostenfeld, 1907, pi. 9, figs. 9, 10a, and b). The plate 

 arrangement in this case is that of fig. 9. Although attributed to the 

 genus, Peridinium, it is doubtful whether, because lacking all acces- 

 sor}^ plates, this form should not be transferred to another genus. 

 No report is known to the writer of the occurrence of a species of 

 Peridinium with one accessory plate. 



Special Considerations. — These lists could doubtless be enlarged 

 by a more complete comparison of published figures and descriptions 

 with material from dinoflagellate collections, were such always ob- 

 tainable. The instances which are given here are taken from the fig- 

 ures which have been drawn with sufficient clearness to serve in this 

 analysis or from the species the plate patterns of which have been 

 confirmed or completed from obsers^ations by the writer. These 

 species selected for mention also represent all the possible categories 

 of plate pattern, ventral and dorsal, and the possible combinations 

 between these patterns as completely as are known. 



It is thus seen that all the six possible combinations between each 

 of the two dorsal symmetrical plate patterns are represented by one 

 or more, usually several species. The ventral plate pattern of the 

 subgenus, Paraperidinium, is known also in combination, but probably 

 as a sport, with one of the asymmetrical dorsal plate patterns, that 

 termed dorsally right oblique. The group Metaperidiwium (dextrad) 

 is known in combination, regularly or exceptionally, with examples of 

 both of the asymmetrical dorsal plate patterns ; and the group included 

 in Orthoperidinium is known, through a single report, in combination 

 with one of the asymmetrical dorsal plate patterns, that termed dor- 

 sally right oblique. 



The fact that we find the only species having but two dorsal acces- 

 sory plates in the group, Orthoperidinium, which, it seems, must be 



