During the Pleistocene, Diamond Valley seems to have been quite a bit wetter than it is now (at least, than it was before it was turned into a reservoir). A fair number of organisms associated with standing water were collected in these deposits, including snails.
The three shells shown above are examples of a species of ram’s horn snail, the ash gyro Gyraulus parvus. Like most of the snails from the Diamond Valley Lake deposits, they are tiny; the black square they’re sitting on is 1 cm on each side.
Modern G. parvus is a widespread species in North America, and is found in a variety of freshwater habitats that involve standing or slow-moving water. According to at least some sources they seem to prefer stable (rather than intermittent) bodies of water. Ash gyros live on the bottom of the pond or attached to water plants, where they feed on benthic diatoms (single-celled algae).