ID help needed from Mexico

[paul monfils]

This is the anterior end of a pen shell, family Pinnidae. It doesn't actually have four lobes. It has two valves, but each of them in this specimen is broken down the middle. These large shells are extremely fragile, and have a very thin outer layer, which is completely eroded away in this specimen, exposing the somewhat thicker pearly inner layer. There is no way of identifying the exact species from such a small and eroded specimen, but since it is from the Sea of Cortez there are five possibilities - Pinna rugosa, Atrina maura, Atrina tuberculosa, Atrina oldroydii, and Atrina texta.