I live in the desert west of Phoenix, and am always finding clamshells in the back yard. I'm pretty sure they're fossils. I was wandering how old they might be.
They are correctly from the family Bivalvia or Pelecypoda. Any shell with two halves.
Clams are found in a lot of areas and they can be recent or fossilized. There are both freshwater and salt water bivalves. If you live near water, like a lake, it could be that critters are pulling them out of the water, eating them and discarding the shells.
Now if they are fossilized they can be weathering out of nearby limestone or clastic rocks, meaning sandstones or siltstones.
Looking at a geologic map, I see that most of the flatlands west of Phoenix have the following description: "Unconsolidated deposits associated with modern fluvial systems. This unit consists primarily of fine-grained, well-sorted sediment on alluvial plains, but also includes gravelly channel, terrace, and alluvial fan deposits on middle and upper piedmonts."
What this means is the flatlands are covered with old river and stream sands and gravels. These are fairly recent and as I said, the shells you are seeing were probably fresh water bivalves, what we call fresh water mullusks but are really just fresh water clams. I used to get them from deep in lakes in Arkansas and eat them.
Now, that is to say that there couldn't be old fossilized shells, but in most cases they would be found incorporated in rocks and not found loose laying on the ground unless they had been replaced by silica and weathered out of a limestone.
I don't think this is the case.
This is what I mean, check this link. http://www.safossils.com/bivalves.html
These are fossil bivalves in southern Arizona. They consist of fossil impressions, as well as casts. The nice black one in the yellow siltstone has been replaced by silica.
Here are a few more with some modern examples thrown in.