Its leaves are alternate, polymorphous, some above and some below the surface of the water, concave in the center whence ribs separate, shield-shaped.
"The Medicinal Plants of the Philippines"
T. H. Pardo de Tavera
It is true that about forty of them range from British to high northern latitudes; but several of them, as, for example, Lutraria rugosa and Pecten polymorphous, which are abundant, are not known at present to range farther north than the coast of Portugal, and seem to indicate a warmer temperature than now prevails on the coast where we find them fossil.
"The Student's Elements of Geology"
Sir Charles Lyell