344 University of California Publications. | ZOOLOGY 
as in H. blackmani. In the form of hypotheca Heterodiniwm 
thus exhibits a development parallel to that found in the allied 
genus Peridinium. The ventral face of the hypotheca is chan- 
neled by the longitudinal furrow plate which in many species 
extends anteriorly so that it indents the epitheca above the 
flagellar pore. 
The girdle is usually submedian in position, though preme- 
dian in some species as for example in H. milneri, or postmedian 
in others as in H. scrippsi. The girdle is much more oblique in 
the flattened than in the rotund forms. In H. whittingae its 
plane is inclined ventro-posteriorly at an angle of 45° to the 
axis. The girdle in all species thus far observed forms a de- 
scending right spiral with a displacement accelerated distally 
and amounting to 1-3 times the width of the furrow. The most 
characteristic feature of the girdle is its incompleteness distally 
and the absence or slight development of the posterior ridge. 
The furrow is bounded anteriorly by a heavy overhanging ridge 
which in species thus far observed is not a ribbed fin or lst but 
a heavy projection of the thecal wall. The posterior border is 
formed by a less salient ridge which becomes less prominent dis- 
tally and often diverges more widely from the anterior ridge 
towards its distal end . The feature of a more or less deficient 
posterior margin of the girdle is a constant character in all 
species thus far observed save the imperfectly known H. sphae- 
roideum which has, however, the ventral plates at least (the 
dorsal ones are not known) of the genus. 
The transverse furrow is indented in the thecal wall and the 
flagellar pore is found at its proximal end. 
The theeal wall is made up of discrete plates, which, however, 
are much less easily separated and much less clearly defined than 
they are in other genera of the family, as for example in Peridi- 
nium. The sutures are marked by flattened ridges or bands or 
smooth tracts in which the cleavage line may be traced in some 
cases in young individuals. The suture bands often have a 
secondary reticulation of minute polygons on their surface and 
are best seen on a deep focus. They are differentiated on the 
inner as well as the outer thecal surface. The epitheca consists 
