Favoritos de bogsnail

I am 100% certain of this location, and the identification seems very certain. I think this might be important, as it is a very rare species only found alive a few times in the last 6 years. the shape matches a species found in Australia. If it's that species, how did it get here? If it's the California native species, could this be an extended range? Introduced? New species, even??

Looks like a butter clam, with some sort of mutation to the shell. I've only ever seen this once in person, but oddly enough there is another observation of another butter clam shell with the same mutation, found in the same place. Maybe a local environmental factor causing irregular growth patterns?