I was wondering if these shells were fossils. If so, how old they might be. I live in the desert west of Phoenix. I find them frequently in our back yard. Thank you.
Seems a bit odd that you are finding them near Phoenix. I would recommend that you take them to a local university and show them to a paleontologist or geologist. Unfortunately this is not an area of expertise for me. Try either Arizona State or the University of Arizona.
If you need to get you a contact let me know.