432 University of California Publications in Zoology [Vol. 18 



in all other respects except that of the dorsal plate pattern corre- 

 sponds with the usual concept for P. claudicans (see pi. 19, figs. 1 

 and 2). 



In addition to these examples just quoted there are a number of 

 other species which a liberal interpretation of the definition of the 

 genus, Peridimum, would include. In all of these additional species, 

 however, the number of dorsal accessory plates is less than three, 

 usually two. These forms also all fall into the group, Orthoperi- 

 dinium and, strangely enough, they are, many of them, inhabitants 

 of fresh water. In most of these forms the two accessorj^ plates are 

 symmetrically placed but may be grouped either as shown in figs. 10, 

 11, or 12. These figures represent all the possible ways in which two 

 accessory plates could be interpolated among the preexisting plates. 



Onl}" one species is definitely reported as having the two accessory 

 plates arranged as in fig. 10 : 



P. quadndens Stein (freshwater) 

 (Stein, 1883, pi. 11, figs. 4, 5, 



and 6) 

 (Schilling, 1913, p. 38, fig. 41) 



Among the species having the two accessory plates arranged as in 



fig. 11 are : 



P. umbonatum Stein (freshwater) P. achromaticum (Lev.) (fresh- 

 (Stein, 1883, pi. 12, figs. 2, 4, wat«r) 



and 6) (Ostenfeld, 1908, pi. 5, figs. 



(Schilling, 1891, figs. 25a 42-43) 



and b) (Schilling, 1913, p. 44, fig. 50) 



P. trochoideum (Stein) Lemm. P. marcliicum Lenim. var. java- 

 (Lemmerniann, 1910, p. 336, nicum Wol. (freshwater) 



figs. 33-36) (Woloszynska, 1912, p. 702, fig. 



P. pusillum (Pen.) (freshwater) 25) 



(Schilling, 1913, p. 41, fig. 45) (Schilling, 1913, p. 42, fig. 47) 



Among the species having the arrangement of the two dorsal 

 accessory plates as shown in fig. 12 are : 



P. laeve Huitf.-Kaas (freshwater) P. thorinnmn Pauls. 



(Huitfeld-Kaas, 1900, pi. '? (Paulsen, 1905, p. 1, fig. 1) 



fig. 3) P. monospinum Pauls. 

 P. aciculiferum Lemm. (fresh- (Paulsen, 1907, p. 12, fig. 11) 



Avater) P. latum Pauls. 

 (Schilling, 1913, p. 39, fig. 42) (Paulsen, 1908, p. 41, fig. 48) 



P. rmiltipunctatum F. F. P. viinutum Kofoid 



(Faure-Freiniet, 1908, p. 227, (Kofoid, 1907, pi. 30, figs. 42- 



fig. 12) 43) 



