Here is a picture of a Trigonia sp. pelecypod fossil at the Museo di Paleontologia at Sapienza University of Rome Italy. Creatures like this existed in the Cretaceous and Jurassic Periods. They had another on display showing the view from the outside.
See some pictures of fossils like I have posted on this blog in the past: http://louisvillefossils.blogspot.com/2009/11/pterotrigonia-thoracica-pelecypod.html
Image taken in June 2014.