This is a tooth from a Ptychodus whipplei that was found in the Atco formation (Cretaceous, Santonian stage)near Prosper Texas.
This smaller tooth is from a different species called P. anonymous. It came from the Arcadia Park formation (Cretaceous, Turonian stage) near Dallas Texas.
Both of these fossils were among a group that I won on Fossil Forum site. Thanks to Travis for sending me these shark teeth.