Blaco River fossils

My wife and I found these fossils in the dry river bed of
the Blaco River in Hays County near San Marcos, Texas.
We haven't been able to find what these fossils are.
They look snaillike but we would like to know more
about them.





Ilymatogyra arietina (Roemer)

Stephen and Marilyn


Stephen, According to the publication "Texas Cretaceous Bivalves and
Localities by the Houston Gem & Mineral Society" would say that the
oysters in your photos are most likely Ilymatogyra arietina, which are
found in the Del Rio Formation. Have found somewhat similar specimens
in the Lake Texoma area. Good Hunting.

These are either Ilymatogyra... or Exogyra. In either case they are the
lower valve of a Cretaceous oyster. Ilymatogyra tend to be smaller and
typical of some of the Texas formations. Exogyra show up in Cretaceous
deposits in more than a dozen states.

Evidently the oysters in question were once called Exogyra arietina (Roemer)
but are designated Ilymatogyra arietina (Roemer) now, as described on p. 72
and shown in photo 193, page 58 in Charles Finsley's Field Guide to Fossils
of Texas. The cork-screw like spiral on top is a diagnostic feature of
Ilymatogyra, and is what earned them the nickname "Ram's horn oysters."

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