Von. 2] Kofoid—On Heterodinium. 343 
ellipsoidal, elongated, rotund, flattened, or even scoop-shaped. 
The length always exceeds either diameter, and the transdiam- 
eter at the girdle equals and more often exceeds the dorso- 
ventral one. The greatest transdiameter is usually at the girdle 
but in some species the epitheca or hypotheca may exhibit a 
slightly greater diameter. The greatest dorso-ventral diameter 
is at the left of the flagellar pore. 
The epitheca is usually not contracted to an apical horn 
though in some species a short horn is present, and in others 
the elongated epitheca tapers gradually from the girdle to the 
apical pore with more or less coneavity of the lateral margins. 
The anterior end of the epitheca is-more often broadly rounded, 
being dome-shaped in the rotund species and like the end of an 
ellipsoid or even seoop-shaped in the flattened species. In some 
instances, as in H. blackmani, the epitheca is rotund at the girdle 
but flattened distally. The altitude of the epitheca is usually 
less than the transdiameter and exceeds it in only a few eases as 
in H. blackmani and H. hindmarchi. 
The ventral face of the epitheca is flattened and somewhat 
excavated, slightly in rotund species, more deeply and. exten- 
sively even to the lateral margins in the dorso-ventrally com- 
pressed forms. The mid ventral face is marked by the slightly 
sinuous suture ridge which runs from the flagellar pore to the 
apex and, bears midway a pit or pore-like area, a characteristic 
structure in the genus. This varies greatly in distinetness and 
in the breadth of the widened smooth suture ridge in which it is 
placed. The apical pore is inclined, even as much as 10° in some 
species, to the right and is usually well defined though rarely 
protuberant. 
The hypotheca is usually subequal to the epitheca, being 
longer in H. milnerit and H. sphaeroideum, and shorter in H. 
blackmani. The posterior end may or may not show a bifurea- 
tion into antapical horns. It may be broadly rounded, dome- 
shaped and without any antapical differentiations as in H. 
sphaeroideum and H. doma, with mere spinules with or without 
lists as in H. milneri and H. murrayi, with slight median bifur- 
cation as in H. whittingae or with typical elongated antapicals 
