I believe that squamosa (Gmelin) and larvata Reeve are synonyms. This shell
has about 18-20 ribs on the upper (right) valve, and about a dozen on the
lower, and may be quite smooth or finely scaly.
squamata (Gmelin) is the heavily scaly one, and has 10-12 main ribs on the
upper valve, and 6 to 8 on the lower.
Paul M.