VoL. 2] Kofoid—On Heterodinium. 349 
Species Latitude Temperature C. 
H. blackmani 7°-380° N. 25°-27° 
H. doma 34°-39° N. 16.1°-18.9° 
H. hindmarchi 19°—39° N. 15..6°=27 .2° 
H. trirostre 262 IN: Dhan 
H. leiorhynchum 19°—39° N. 15.5°-24.5° 
H. murrayi 14°-28° N. 20°-25.5° 
H. milneri 29°-31° N. 26.9°-27.2° 
H. sphaeroideum 
ier ea San Diego ee 
Bly arcane B45 (1 ANTE 14.6°-22.5 
H. inaequale 
H. triacantha 55°=8il> N. 
Their vertical distribution is not known. At San Diego no 
individuals have been taken in the many surface catches of the 
tow nets made during the past few years. They have been found 
only in the vertical catches in from 165 to 40 fathoms to the 
surface. Murray and Whitting’s (’99) species were all appar- 
ently from plankton collected by filtermg water from ship’s 
pumps and therefore taken some 2-3 fathoms below the surface. 
The absence of chromatophores or their aggregation in chromo- 
spheres observed in individuals taken at San Diego is suggestive 
of occurrence in deep water with diminished light. The extreme 
hyalinity of some species is also indicative of a deeper habitat. 
The excessive development of the reticulum on the theca, and 
the asymmetry are evidently adaptations for flotation, on the 
one hand by inerease of friction surface, which is at least doubled 
in the more rugose forms, and on the other by giving a spiral 
course to any passive descent of the organism due to gravity, and 
thus prolonging its existence in the upper strata. 
This is a genus of somewhat aberrant structure and is repre- 
sented by relatively very few individuals in comparison with 
those of Ceratium and Peridimum. I regard it as a degenerate 
form unable to maintain itself at the surface and for some reason 
deficient in reproductive vigor. In comparison with the number 
of individuals observed the number of species is large. The 
known species are all well defined and observations on difterent 
individuals do not indicate as yet any noticeable intergradation. 
