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 graciUs Leidy. This unique species is found in some 

 abundance on the projecting margins of the shells of the Unionida 

 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. 



