Bacopa salzmannii - aka
Bacopa sp. 'Araguaia'. Thanks to AaronT for the specimens. A name at last!
http://www.aquaticplantcentral.com/f...us&spec=Bacopa Hemianthus micranthemoides - updated habitat information to make it more accurate. Not seen in nature since 1941, it is/was a plant of tidal rivers, not ponds. With a flowering specimen provided by Delaware Jim and some help from a botanist, I have been able to confirm that the plant in the hobby is indeed
H. micranthemoides.
http://www.aquaticplantcentral.com/f...ails.php?id=33