VoL. 2] Kofoid.—On Heterodiniwm. 347 
often feebly expressed. The antapical series includes 3 plates, 
a single dorsal (18), a median ventral (19), and a left ventral 
(20). The distal end of the posterior list of the girdle usually 
descends on the suture between the right ventral and right latero- 
ventral postmedians and continues to the tip of the right antap- 
ical horn. 
The boundaries of the plates on the right ventral face of the 
hypotheeca are subject to much obscurity and considerable varia- 
tion in loeation. The relations of the midventral plate (19) to 
( 
are subject to considerable variation in connection with the vary- 
the adjacent postmedian (17) and to the longitudinal furrow 
ing degrees of development of the posterior list of the girdle. 
The longitudinal furrow is subject to considerable variation 
in length. It is relatively short in some species as in H. rigdenae, 
H. sphaeroideum, and H. hindmarchi where it is less than two- 
thirds the length of the hypotheea. In other forms as H. tri- 
rostre and H. murrayi it reaches the antapical border. It is 
without high membranous lists as a rule though one appears in 
HH. murray. 
The thecal wall is thin and hyaline and universally, except 
in apparently young individuals, reticulate with more or less 
irregular polygons formed by thickened ridges on the outer sur- 
face. In some species, as in H. scrippsi, and perhaps in certain 
stages of growth of other species these polygonal boundaries be- 
come so prominent as to obscure suture lines, as may be seen in 
Murray and Whitting’s (’99) figures of H. trirostre, H. murrayt, 
and H. hindmarchi. 
These reticulations are often quite regular as in H. blackmani 
and along the lateral margins of the epitheea of H. scrippsi, or 
very irregular as on the ventral and dorsal faces of H. scrippsi 
and in H. trirostre ; they may be relatively large as in H. murrayi 
or small as in H. sphaeroideum, with very heavy ridges as in 
H. trirostre or but faintly outlined as in H. milneri, or forming 
but a delicate network as in H. blackman. In young individuals 
they may be entirely lacking and the presumption is that in gen- 
eral, individuals with partially or feebly developed reticulations 
have not as yet reached the stage of completed formation of the 
