I am not familiar enough with the
Update 6/6/16: A vigilant reader pointed out that the genera Gryphaea is restricted to Europe and that the equivalent in the Cretaceous of Texas/Oklahoma is Texigryphea. He also recommended an old issue of "Oklahoma Geology Notes", Vol 35, No. 2 which has an paper by Robert Fay titled "The Type Species of Mortoniceras and the Holotype Specimens of the Lower Cretaceous Texigryphaea of the Southwestern United States". It has some helpful information and a key that may be used to ID specimens of Texigryphaea. The link above takes you directly to the paper but if you click here you can go to the Oklahoma Geological Survey site and look through all 74 volumes (up to winter of 2014). There are a lot of good publications and information on their website.
Left valve top
The above specimen comes from the Duck Creek formation which is Cretaceous (Albian stage) in age.