Rhodymenia palmata:

Rhodymenia palmata (L.) Grev., Palmariaceae: See Palmaria palmata (L.) Kuntze.

Rhodymenia palmetta:

Rhodymenia palmetta (J. V. Lam.) Grev., Palmariaceae: See Rhodymenia pseudopalmata (J.V. Lam.) Silva.

Rhodymenia pseudopalmata

Rhodymenia pseudopalmata (J. V. Lam.) Silva.
Bot. syn.: Rhodymenia palmetta (J. V. Lam.) Grev.

I have no articles for this botanical term, but there might be some for the next level down (if found).