342 University of California Publications. [ ZOOLOGY 
plankton which brought to light the most of the species here dis- 
cussed. The types and cotypes of the species here deseribed for 
the first time are in the collections of the University of California. 
Heterodinium gen. nov. 
The form of the theca resembles that of Peridinium in the 
presence of a median or somewhat postmedian girdle which en- 
circles the theca at its greatest diameter and in two antapical 
horns, always directed posteriorly or nearly so. The posterior 
margin or lst of the transverse furrow is suppressed or feebly 
developed in comparison with the anterior one, especially at its 
distal end, while the anterior one, as if in compensation, is often 
excessively developed. The suture lines are demonstrated with 
great difficulty and some uncertainty in most of the species. 
The plates are as follows: three apicals, one left interealary, six 
premedians, seven postmedians, one furrow plate, and three ( ?) 
antapieals, as shown in the accompanying text figures. 
On the ventral face about midway between the apical and 
flagellar pores is a small pit or pore-lke area in the mid ventral 
suture. An actual opening in this area has not been demon- 
strated. In the suppression of the posterior border of the girdle, 
in the number and arrangement of the plates and in the presence 
of the ventral pit on the epitheea Heterodinium differs from 
Peridinium, although in form and general appearance species of 
the two genera strongly resemble each other. 
In some species and possibly generally in the genus there is 
a decided asymmetry to the theea brought about by a torsion of 
the body on the main axis in clockwise direction, looking from 
the posterior toward the anterior end. This is especially notice- 
able in the scoop-shaped forms such as H. scrippst. 
DETAILED DESCRIPTION. 
The following is a more detailed description of the characters 
found in the genus. The theca is expanded in the equatorial 
region, with more or less well marked dorso-ventral flattening 
and ventral excavation near the flagellar pore. It is spheroidal, 
