Which plant is the only genus in the family, Ceratophyllum or Conoclinium?
Ceratophyllum: Ceratophyllum is a cosmopolitan genus of flowering plants including four accepted species in 2016, commonly found in ponds, marshes, and quiet streams in tropical and in temperate regions. Ceratophyllum:  It is the only genus in the family Ceratophyllaceae, itself the only family in the order Ceratophyllales. Conoclinium: Conoclinium, the mistflowers, is a genus of four species of herbaceous perennial flowering plants, native to North America.
Ceratophyllum