Figure 4. a–l. Line drawing of the thecal morphology of someProtoperidinium species in section Oceanica . Fig. a–d.Protoperidinium vietnamicum sp. nov.: cell in ventral view (a), dorsal view (b), right lateral view showing the oblique cingulum (c), and the patterns of the three intercalary plates and the plate 4” (d); Fig. e–h. Protoperidinium murrayi (Kofoid) Hernández-Becerril (1991, p. 79): cell morphology in ventral view (e), dorsal view (f), right lateral view (g), the patterns of the three intercalary plates and the plate 4” (h). Fig. i–l. Protoperidinium curvicorne comb. nov.: cell morphology in ventral view (i), dorsal view (j), left lateral view showing curved cell relative to the longitudinal axis by the apical horn strongly tilted forward, dorso-ventrally compressed cell and strongly oblique cingulum (k), the patterns of the three intercalary plates and the plate 4” (l). Scale bar in Fig. a applies to Figs b–c; the scale bar in Fig. e applies to Figs f–g, and the scale bar in Fig. i applies to Figs j–k.