I am looking for information or identification of something I found as a child here in Idaho. It looks very similar to a horseshoe crab and I used to find it in a shallow body of water near my hometown of Rexburg. It was the size of a quarter and the shell was not completely hard like that of a clam but more like a fingernail. It was dark reddish brown in color and the shell was slightly transparent as it was not that thick. I remember being afraid to touch it because it was scary looking because of the sharp looking pointy tail. I was afraid it might sting me or something. I remember flipping it over on the shore with a stick to see the underside. I do not remember exactly how many legs it had but it looked much like the horseshoe crabs. I remember that the shell on top covered the entire body and the only thing that stuck out the back was the tail. I do not remember any eyes and the shell was very smooth. I mentioned it to someone at work recently and they had never heard of such a thing I so I am just wondering if anyone has any clue what this might be. Or where I might look to find out what it is. I am going to try to find one again. The body of water where I found them is gone but there is another close by and fairly similar environment. Hopefully they are still around. Thanks for any suggestions you may have.