346 University of California Publications. | ZOOLOGY 
and one each right (9) and left (6). They are not as a rule 
symmetrically placed because of the considerable irregularity in 
their size and especially because of the disturbing effect of the 
left intercalary plate (4) which is found between premedians 
5, 6, 7 (or 6 and 7) and apicals 1 and 3. This interealary plate 
is often small and in compressed species is not easily found. It 
is shown clearly in H. blackmani, but is merely suggested or not 
shown at all in the rest of Murray and Whitting’s (’99) species. 
Its demonstration in all species carefully examined by me leads 
me to expect it in others especially since they usually show the 
tilting of the apical pore to the right, the slight shoulder on the 
left marginal outline and the asymmetrical arrangement of the 
premedians which attend its presence in species in which it has 
been demonstrated. It is greatly reduced in size in flattened 
species such as H. whittingae, and it is certainly possible that 
this plate may be entirely suppressed in some of the flattened 
species of the subgenus Platydinium, though no conclusive evi- 
dence to that effect is at hand. 
The ventral face of the epitheea of the flattened species is 
formed by the two ventral-apical plates (2, 3) and the two 
ventral-premedians (5, 10) but in the more rotund forms the 
lateral-premedians (6,9) are partially exposed in a ventral view. 
The left interealary in the flattened species is dorsal in position, 
but in the rotund forms as H. murrayi and H. doma it appears 
on the left shoulder in the ventral view, and may be shifted 
dorsalwards so that it does not touch the ventral premedian 5. 
The girdle plate has the form of a trough-like band as in 
Peridinium and appears to be variously subdivided by reticular 
ridges some of which may be suture lines. 
The plates of the hypotheea are less clearly defined than 
those of the epitheca, especially on the ventral face. There are 
seven postmedians (11-17) adjacent to the girdle, 3 dorsal 
(13-15) and 4 ventral (11, 12, 16, 17) one of which (16) appears 
to extend to the tip of the right antapical without subdivision 
into anterior postmedian and posterior antapical moieties, ex- 
cept possibly in H. sphaeroideum. The separation of the adja- 
eent postmedian (17) from the ventral median antapical is 
