1918] Barrows: Skeletal Variations in the Genus Peridinium 435 



and 41, figs. 9 and 10) with the ventral plate pattern of Joergensen 's 

 subgenus Metaperidinium. 



2. P. granii Pauls, has been figured by Paulsen (1907, p. 15, 

 fig. 18) with the ventral plate patterns of Paraperidinium and the 

 asymmetrical right oblique dorsal pattern of fig. 7. Wliat is presum- 

 ably this same species has been found in Sausalito Bay, California, 

 with the Metaperidinium pattern for the ventral plates and with the 

 asymmetrical left oblique dorsal pattern of fig. 8 ; and in still a third 

 condition with the right oblique dorsal plate pattern and pattern of 

 Metaperidinium for the ventral plates in forms from Union Bay, 

 Alaska (pi. 17, figs. 1 and 2; and pi. 18, figs. 3 and 4). 



3. P. claudioans Pauls, is usually found with the ventral plate 

 pattern of Orthoperidinium and the primary sj-mmetrical dorsal plate 

 pattern of fig. 5. However, specimens from Sausalito, California, pre- 

 sent a dorsal pattern of the right oblique type (fig. 7, pi. 19, figs. 5 

 and 6). 



4. In the same Saiisalito collection as that just referred to was 

 also found a specimen of what was probably a deformed race of 

 P. oceanicum Van H., which was remarkable in displaying the ventral 

 epithecal plate pattern of the subgenus Paraperidrndum instead of the 

 usual pattern for P. oceandcum Van H., that of Orthoperidinium 

 (pi. 20, figs. 7 and 8). 



5. The species P. divergens Ehrbg. is usually found with the 

 ventral plate pattern of Metaperidinium and the primary sjnnmetri- 

 cal plate pattern of fig. 5. Two specimens out of sixty-three exam- 

 ined by the writer have been found with a ventral plate pattern of 

 Paraperidinium, and this combination has also been figured by Mangin 

 (1910, pi. 7, figs. 10 and 13). 



Summary of Skeletal Relationships. — We are dealing then with 

 the skeleton, the most permanent of the structures of this organism, 

 but a structure which undergoes certain modification even in the life 

 of the individual. From genus to genus within the family, Peridi- 

 nidae, we find this skeleton undergoing variations in regard to the 

 number of its plates. We find that in this family the number of 

 plates is increased first by additions in one or another of four funda- 

 mental rows. In this progressive increase in number of plates the 

 antapical row usually contains the least number of plates, often 

 not more than two. The apical row contains more plates than the 

 antapical but usualh^ fewer plates than either of the cingular rows. 

 Of these cingular rows it is the precingular in which the number of 



