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What type of shell is this? How old?


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I found this shell at half moon bay in Northern California, It washed up on shore. Im pretty sure it’s a fossil - I’m wondering what type of shell it is, and possibly how old it could be? Any help would be much appreciated! 

Approx dimensions: L3” x W2.5” x H1.5” 






Edited by Naterskater
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Tidgy's Dad

Maybe a species of Crepidula? 

Here's an article on the geology by our own @Boesse

https://ucmp.berkeley.edu/2010/05/marine-vertebrate-paleontology-in-half-moon-bay/#:~:text=In the Santa Cruz area%2C fossils of sharks%2C rays%2C,spot in Half Moon Bay.


So, it's Miocene / Pliocene about 7 to 2.5 million years old. 


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Indeed, a Crepidula princeps from the Purisima Fm.

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