Tryblidiida

Veleropilina reticulata (Seguenza, 1876) ("Monoplacophora" : Tryblidiida: Tryblidioidea: Neopilinidae), Tuscan Arcipelago, depth of 500 meters Photo by Andrea Nappo, for World Register of Marine Species, Creative Commons Non-Commercial Attribution via Encylopedia of Life.

The Tryblidiida are a group of very primitive and ancient mollusks with mostly simple cap-shaped shells. They have previously been (and are often still) called the Monoplacophora. However, it is now known that "Monoplacophora" refers to a very diverse assemblage of basal mollusks, and is not a natural group. John Peel suggest that the Monoplacophora should be divided into two classes, the Tergomya and the Helcionelloida, but even the Tergomya may be a polyphyletic group.