1918] Barrotvs: Skeletal Variations in the Genus Peridiniimi 447 



The combinations of the asymmetrical dorsal plate patterns with 

 the ventral plate patterns are less complete, possibly because less 

 fnlly known, and it is to be expected that under certain circumstances 

 the missing combinations may still be found occasionally, with the 

 ever-increasing intensity of our examination of this group. At present 

 the ventral plate pattern of the subgenus, Metaperidinium, is known 

 to occur in combination with both of the dorsal symmetrical plate 

 patterns, and at least one instance, in the case of P. punctutatum 

 Pauls., is known for the combination of the ventral plate pattern of 

 the subgenus, Ortlioperidinium, with the right oblique asymmetrical 

 dorsal pattern. 



That portion of the genus having but two accessory plates is alto- 

 gether confined to the subgenus, Orthoperidinium. This pair of acces- 

 sory plates seems to have appeared in two methods : one, that shown in 

 fig. 10, seems to have been able to proceed no further than this first 

 stage; the other, shown in fig. 11, with increase in size of this pair of 

 accessory plates, might have formed a pattern such as that of fig. 12, 

 from which the full complement of three accessory plates as shown 

 in figs. 5 and 6 might have developed. All of these combinations of 

 symmetrical dorsal and ventral figures are known in Orthoperidinium, 

 and also combinations with various asymmetrical patterns. 



The unexpected combination of an early or primitive type of 

 dorsal pattern, involving but two accessory plates, with a late or 

 specialized type of ventral pattern, that of the subgenus, Orthoperi- 

 dinium, may perhaps be explained for some of these species on the 

 basis that many of these forms are derived from brackish water or 

 marine progenitors, and having undergone an unusual transition upon 

 being transferred to a body of fresh water, an intrinsic subordination 

 of the introduction of dorsal accessory plates to the accession of ven- 

 tral precingular plates may have become accentuated. The fact that 

 nearly all freshwater species have the combination of plate patterns 

 first described may be accounted for upon the basis that species of 

 the subgenus, Orthoperidiwium, are much more common than those of 

 either of the other subgenera of Peridinium, and furnish the more 

 frequent chance for accidental transfer. In such a form as P. excen- 

 tricum, having one small and one very large accessory plate, it is not 

 impossible that a secondary fusion may have occurred between the 

 middle and right accessory plates. 



