348 University of Californa Publications. [ ZooLoGy 
theea. The reticulations are found also on the girdle plate and 
on the girdle lists and along suture lines there are frequently 
rows of smaller polygons. In but a few cases as in H. murrayi 
and H. doma are enclosing ridges so thickened by the filling in 
of the angles as to leave a subcireular central area. Each reticu- 
lation has typically one centrally located pore. In some species 
with coarse reticulations there are several pores in a single area, 
and frequently in all species there are minor irregularities in 
the number and position of the pores. Small polygons fre- 
quently lack the pores. The reticulation is evidently formed on 
the outer surface of the theeal wall by plasma which is extruded 
through the pores, for the polygons bear a definite relation to 
the arrangements and distribution of the pores. 
The protoplasmic contents of the theca are usually hyaline 
and colorless, and often only partially fill the interior of the 
theea. The nucleus has the usual ellipsoidal form with beaded 
chromatin reticulum and lies near the center of the protoplasmic 
mass not far from the flagellar pore. It is small and is found 
with difficulty. No instances of diffuse reddish coloration often 
seen in Peridinitum and Pyrophacus have been noted as yet in 
Heterodinium. Chromatophores are entirely absent in some in- 
stances, in others they are massed in spheroidal chromospheres 
of pale greenish yellow or deep cadmium orange color. In some 
eases the chromatophores are peripheral in location and of va- 
rious forms. Vacuoles and pusules of varying form and distribu- 
tion have been observed in the cell contents. 
The dimensions of observed species are like those of Peridi- 
nium. The largest species thus far recorded appears to be H. 
blackmani which has a length of 225y, and transverse and dorso- 
ventral diameters of 1354 and 160» respectively; the smallest 
appears to be H. sphaeroideum with a length of only 42u, and 
transdiameter of 39x. 
The distribution of this genus from species thus far pub- 
lished appears to be limited, in the main, to warmer seas as shown 
in the following table: 
