Lamarck separated the mother-of-pearls shell (MARGARITA) from the swallow-tail muscles (AVICULA) on account of its more orbicular shape.
"Journals Of Expeditions Of Discovery Into Central Australia And Overland" by Edward John Eyre
The Obydos shells are not marine, but are fresh-water Unios, greatly resembling Aviculas, Solens, and Arcas.
"The Atlantic Monthly, Volume 18, No. 106, August, 1866" by Various