My friend from Pennsylvania left a number of fossils for me to study. Here is one that is a Devonian pelecypod found in Mahantango Formation located in Perry County, Pennsylvania. The rock seems like some sort of shale? The Mahantango Formation is named for part of the Mahantango creek found in Pennsylvania and is a member of Hamilton Group.
Maybe this pelecypod is of the genus Nucula or Paracyclas? Here is a neat web page on Pennsylvania fossils.