290 
until minimum temperatures are reached, when minimum numbers 
appear in the plankton. As temperatures rise, the statoblasts 
tend to float and become more abundant in the plankton, as a result, 
perhaps, of the physiological and accompanying physical changes 
in the contents of the statoblast. The declining phase of the major 
flood of the year is thus the period of greatest flotation and dispersal 
of the statoblasts. 
Urnatella gracilis Leidy.—This unique species is found in some 
abundance on the projecting margins of the shells of the Uniomde 
which line the river bottom in many reaches of the channel. Small 
fragments of the colonies containing only several polypides were 
found in the plankton in May—August and October. The earliest 
record was May 25, and the latest, October 25, at 48.5°. 
