After I remarked that the little tooth below (found on Morris Island, South Carolina) looked interesting on his trip report thread, forum member @Family Fun kindly gifted it to me, asking only that I help identify the species and share the result. My assessment is that this is an Isurus retroflexus posterior tooth, but hopefully others here can either confirm or correct that ID. This tooth has a non-serrated crown that is still fairly sharp and to my eyes at least has the raised labial platform indicative of I. retroflexus. There is a single cusplet on one side.
Thanks for your help with this one, and thank you @Family Fun for your generosity!
(Unfortunately, the tooth split down the middle on its journey to me, which is the crack you can see in the photos, but I was able to glue it back together.)