Another oyster found in the Cretaceous sediments of Texas is this curled specimen. Below is what I believe is a Ilymatogyra arietina based on the book "A Field Guide to Fossils of Texas" by Charles Finsley, plate 58, photo 192.
This fossil comes from the Lower Cretaceous aged Grayson Fm., Denton County, Texas.
As a comparison, here are the three Texas oysters that I've detailed in the last few posts side by side. From left to right: Texigryphaea navia, Texigryphaea marcoui, Ilymatogyra arietina