From The Daily Astorian, April 29, 1879:
• We have all heard fish stories, but of all the interesting stories we ever listened to, none can compare with Capt. Manson's clam story of San Pedro, California. The clams are there by the millions, and they actually walk (or hop), and swim on the water, by flapping the shell. Fact.
Note: The Ear, who was brought up in an oystering village on the East Coast, at first thought this was a whimsical bit of nonsense. But it's not. And those clams he saw were likely scallops, which are also bivalve mollusks.
Scallops actually do swim in — not on the water — and at a pretty good clip. They can propel themselves both forward (swimming) and backward (jumping) by sucking in and expelling water.
You can see a scallop in action at bit.ly/goscallop (a screenshot is shown). They rarely go more than about a yard up off the sea floor, and seldom for more than about 16 feet, but nonetheless that's pretty impressive.
Other scallop notable features: They can have up to 200 eyes along the edges of their shells and they can produce pearls. And yes, there are scallops at San Pedro, even now.