Pictures shown in this posting are from Cretaceous Period (145-65 million years ago). The fossils were found in the Fort Worth, Texas area.
This first tooth belonged to the Cretoxyrhina mantelli (aka Ginsu Shark). Learn more on Wikipedia about it at this entry. According to that article its diet consisted of turtles, Mosasaurs, Plesiosaurs, and Xiphactinus.
These next fossil teeth are of the extinct hybodontiformes Ptychodus anonymus shark. Looking at the teeth it appears to be a shell crusher.
Another set of shell crushing fossil teeth of the Ptychodus whipplei shark.
This fossil shark tooth belonged to the Scapanorhynchus (aka Spade Snout) shark. According to this Wikipedia entry, it is similar to the modern goblin shark (Mitsukurina owstoni).
This last shark tooth belonged to a Protolama sokolov shark.
Thanks to Herb for the pictures.