1918] Barrows: Skeletal Variations in the Genus Peridinium 409 



placement of the ventral ends of the girdle; and (3) the suture pat- 

 terns formed by the varied arrangements assumed by the plates of 

 the shell. 



Type of Antapioal Horn. — From the prominence which the ant- 

 apical horns assume in many species of the genus, one is tempted to 



© (D 



Para peri d I n ium 



Mefaperidinium OrihoperfJinium 



\l'y 



2' V 4' 



© ® (D 



P Ccnicum Ricjhf Obliaue Left Ob/if ue 



VARIATIONS IN PLATE PATTERN OF PERIDINIUM 



Figs. 1-16. — Diagrams of the arrangement of the ventral plates of the 

 epitheca in the three subgenera of Peridinium and for the several arrangements 

 of the dorsal plates, showing in the series of figures from 9 to 12 the two 

 probable methods for the introduction of the accessory plates and the develop- 

 ment of the accessory row to the three-plate stage. Figures 13-16 indicate in 

 the small triangles the regions at which the early accessory plates may have 

 been introduced. (Figures 2, 13, 14, 15 and 16 have not been authentically 

 reported in specimens). Figure 4b, is only a variation of the pattern for the 

 subgenus Ortho peridinium, as given in figure 4a. 



