Fig. 7. Ostreopsis siamensis Schmidt. Sketch of a specimen in lateral view. 
Arrangement of plates not indicated. 
In its shape this peculiar genus is nearest to Pyrophacus Stein of 
the genera hitherto known; the number of apical- and antapical plates 
reminds of Gonyaulax Diesing, but from both those genera it is easily 
distinguished by the above features. 
23. 0. siamensis Schm. n. sp., figg. 5, 6, 7. 
Body flat, oyster-shaped, somewhat convexo-concave, in a trans- 
versal section view triangular or roundish. Superior valve convexe, 
with 3 apical plates (one of which being very small) and 7 pre- 
median plates; inferior valve a little concave, with 1 antapical and 
4 larger postmedian plates (and sometimes with small accessory 
plates). Longitudinal girdle short, only on the inferior valve. 
Structure coarsely porous, like that of Ceratium tripos. Length of 
sagittal axis about 90 p. 
2(r) — 3(r) — 6(rr). 
Peridinium Ehrenberg. 
24. YP, divergens Ehrenberg. 
This species, which appears in large quantity in the Gulf of Siam, 
varies exceedingly much in regard to size, shape and length of the po- 
sterior horns. 
1(+) — 2) — 3(4+) — 407) — 5) — 6(+) — 7(+) — 10(4). 
Area; Ubiquitous. 
25. P. conicum (Gran) Ostenfeld & Schmidt |. c., p.174; P. divergens 
var. conica Gran, Hydrographic-Biologic Studies of the North-Atlantic 
Ocean and the Coast of Nordland, 1900, p.47; Fig. Bergh l.c., Pl. 
XV, f.43—44; Pouchet l.c., Contributions J, fig. 31—33; Schiitt 1. c. 
PL. XU, f. 4313, 14. : 
1 (rr) —- 6(r) — 7 (rr) — 10(r). 
Area: Atlantic, Mediterranean, Red Sea, Indian Ocean. 
26. P. oceanicum Vanhdéffen, in Drygalski, Grénland-Expedition der 
Gesellsch. fiir Erdkunde zu Berlin, vol. II, 2 part, Pl. V, fig. 2. 
Q(rr) — 6 (rr) — 7 (rr) — 10 (tr). 
Area: Atlantic, Red Sea, Indian Ocean. 
27. P. elegans Cleve, Notes on some Atlantic Plankton-Organisms, 
Kel. Sv. Vet.-Ak. Handl., Bd. 34, No. 1, p. 16, Pl. VI, fig. 15— 16, 
3 (rr) — 6 (rr). 
Area: Warm Atlantic, Red Sea, Indian Ocean. 
