350 University of Californa Publications. [ ZooLocy 
The nearest allies of this genus are plainly in the family 
Peridinidae, though it shows no marked structural affinities to 
any particular genus. The form cycle found in its species re- 
sembles that of Peridinium but its thecal plates are entirely 
different. The midventral diamond-shaped plate of the epitheca 
so characteristic of Peridinium is entirely lacking in Heterodi- 
nium, unless indeed the slight midventral expansion on the 
suture line be taken to represent a degenerate midventral which 
seems improbable. The excessive development of reticulations 
on the surface of the theca approaches that in Protoceratium 
but this genus appears to lack the midventral pit on the epitheca, 
and has a narrow transverse furrow which is complete distally. 
Its plates (see Schutt (’96)) are not known and it may prove 
to have a closer relationship to Heterodinitum when these are 
definitely determined. 
The midventral pit on the epitheea of Heterodinium resembles 
the so-called ‘‘pore’’ in Poroceratium gravidum (Gourret) but 
bears a different relation to the thecal plates and is probably not 
a homologous structure. In Poroceratium the ‘‘pore’’ hes near 
the middorso-ventral line in the middle of the dorsal and ventral 
apical plates, whereas in Heterodinium it lies in the suture be- 
tween the two ventral apicals. 
The generic distinctness of Heterodinium is thus beyond ques- 
tion and it belongs with Ceratium, Peridinium and Protoceratium 
in the sub-family Ceratiinae. 
SYNOPTIC KEY TO THE SPECIES OF HETERODINIUM. 
Sphaerodinium subgen. nov. 
Body spheroidal, antapical horns not present or only slightly 
developed as spines. Epitheca rotund without stout lateral 
sutures. 
ft; With nosamntapreall horns: Or (Sp Une St sete cae eee sgvtzeteacs 2 
ie SWithantapicall’ spines yas oss es eee ees eee See ieee oe 3 
2. Outline smooth, sutures faint, reticulations minute..H. sphaeroideum 
2. Outline subangular, sutures prominent, reticulations coarse..H. doma 
3. No apical horn, thecal markings faint S22 e ee H. milneri 
3. Short apical horn, thecal reticulation prominent .............. H. murrayi 
Bp) slong apical) horns sects ene eee ee eee H. triacantha 
