VoL. 2] Kofoid.—On Heterodinium. ~ 345 
of ten plates (figs. A and B) arranged as follows: three apicals 
about the apical pore, of which one (1) is a wide dorsal plate 
covering the dorsal half of the anterior end, and the other two 
(2,3) are ventral and are separated from each other by the mid- 
ventral suture which runs from the apical to the flagellar pore. 
Between the dorsal and the two ventral plates are lateral sutures 
which in subgenera Platydinium and Euheterodinium become 
very heavy and much more prominent than any other sutures 
Fig. A.—Ventral view of Heterodinium scrippsi showing thecal plates; 
apicals, 1-3; premedians, 5-10; left intercalary, 4; postmedians, 11- 
17; antapicals, 18-20. x 450. 
Fig. B.—Dorsal view of same. X 450. 
of the theea. In some eases as in H. scrippsi and H. blackman 
these lateral ridges are doubled, suggesting a narrow compressed 
plate, but in the more rotund species they are not doubled and 
there is no suggestion of the presence of such a plate. I there- 
fore conclude that they are merely doubled margins and are not 
to be regarded as the margins of degenerate plates. 
Anterior to the girdle is a premedian series (figs. A, B, 5—10) 
of 6 plates of which two are ventral (5, 10), two dorsal (7, 8), 
