Ceratopteris cornuta is called the water sprite and looks like the image(s) pictured above. There were four recognized species: C. thalictroides, C. cornuta, C. pteridoides, and C. richardii. Some taxonomists think there is only one species that is highly variable in growth form. (Lloyd, 1974 - Systematics of the genus Ceratopteris Brongn. (Parkeriaceae)).
Nemosreef, could you point me to a taxonomic reference for the specific species you are referencing? My taxonomy info is probably out-of-date.