1918] Barrows: Skeletal Variations in the Genus Peridinium 429 



species as included in the primaty group, Paraperidinium. Species 

 presenting the dorsal plate pattern of fig. 6 might be termed as con- 

 stituting a group of Paraperidinium secondary. 



P. ovatum Pouchet P. tristylum Stein 



(Faure-Fremiet, 1908, p. 219, (Stein, 1883, pi. 9, figs. 15 and 



fig. 5) 16) 



P. cerasus Pauls. (Broch, 1910, p. 187, fig. 5) 



(Meunier, 1910, pi. 2, fig. 27) 



These species have not been foimd by the writer but two instances 

 of what otherwise was clearly P. divergens Ehrbg. have been found in 

 which the ventral pattern of Paraperidinium was present. Mangin 

 (1911, pi. 7, figs. 10 and 13) has also figured P. divergens Ehrbg. in 

 which this unusual pattern has occurred. 



A specimen taken to have been derived from a race of P. oceani- 

 cum Vanh. from Sausalito, California, also shows the ventral plate 

 pattern of Paraperidinium with the primary dorsal plate pattern of 

 fig. 5. This combination in this case is explained as due to unusual 

 physical conditions of Sausalito Bay, which have, so to speak, de- 

 formed this species characteristic of the open sea. 



Species of Paraperidinium bearing the dorsal plate pattern of 

 fig. 6 and which may be considered as constituting a group of Para- 

 peridinium secondary, include : 



P. paUidum Ost. P. pellucidum (Berg) var. acutum 



(Broch, 1910, p. 45, fig. 17) F. F. 



P. islandicum Pauls. (Faure-Fremiet, 1908, p. 221, 



(Broch, 1910, p. 46, fig. 20) fig. 7) 



(Paulsen, 1904, p. 23, fig. 7) P. spinoswn, Schiller 



P. pellucidum (Bergh) Schlitt (Schiller, 1911, p. 3, fig. 3) 



(Paulsen, 1908, p. 49, fig. 62) P. curvii^es, Ostf. 



P. pellucidum (Bergh) var. eras- (Paulsen, 1911, p. 308, fig. 6) 



sum F. F. P- rectum Kofoid. 



(Faure-Fremiet, 1908, p. 220, (Pavillard, 1916, p. 40, fig. 17) 



fig. 6) P 



This combination has been confirmed in a number of specimens 

 of P. pellucidum found by the writer. 



The only example which we have met of a combination of the 

 ventral pattern of Paraperidinium with one of the asymmetrical 

 dorsal patterns is a figure by Paulsen (1907, p. 15, fig. 18) for 

 P. granii Pauls., which agrees well enough with the general concept 

 for P. granii in other respects, though specimens of this species which 

 we have seen from Yes Bay, Alaska, carry the ventral plate pattern 

 of Metaperidinium with the asymmetrical right oblique dorsal pattern. 



