Laevapex fuscus, muscle scars. The adductor muscles are inserted into the under surface of the shell. The muscle attachment scars are shown here. In Laevapex, there are additional smaller muscles attaching to the shell, a series between the two anterior adductor muscles, and a series between the right anterior adductor muscle and the posterior adductor.
John B. Burch (photographer; copyright holder), Mollusk Division, Museum of Zoology, University of Michigan-Ann Arbor