While the Diamond Valley Lake fossil fauna is best known for its mammals, there were also thousands of mollusks recovered. These are mostly minute freshwater species, and even though we have thousands of them the all fit easily in a single specimen case.
While most of these mollusks are gastropods (snails), there are some bivalves as well. Above is the right valve of a pea clam, Pisidium, shown in lateral or exterior view. The alternating black and white lines in the background are each 1 mm wide. Below is the same shell in interior view:
The shell’s hinge, where the left valve would have attached, is visible along the lower left margin of the shell. Inside the shell, you can just make out the adductor scar; this is the attachment point for the muscles that close the shell.
As I’ve been discovering, it’s difficult to find a lot of information online about freshwater mollusks. Pisidium is a widespread extant genus, found in (at least) Eurasia, Africa, and North America. It appears that many of the modern species found in North America are invasive species from Europe, but clearly the genus was present here long before the arrival of Europeans.