Photo Juergen Buchelt
Photo Guido Zsilavecz. A 50 cent coin in the middle can be used to judge size.
On both Blouberg beach and close to the lagoon mouth at Milnerton, teeth such as those shown can be found by those with the patience to look for them. The northern (right-hand-side) sand spit leading out to the rocks, at Big Bay, especially after a winter storm when the sea has calmed down and the tide is low, is a good place to search for them. It takes a while for the eye to distinguish the dark, moderately dull grey teeth from the very similar-looking mussel shells, but once you see one finding others becomes easier. The actual origin of the teeth is not known - I read somewhere that it had to do with the escavation of the "harbour" at Koeberg, with the teeth carried south to Blouberg. They are slightly less dense that pebbles and stones, so are moved with more ease by currents.
From what species the teeth originate we do not know. The larger teeth look like they might come from an ancestor of the great white, including the very large Carcharodon megalodon, but others look more like they may be from mako ancestors (Isurus oxyrinchus). As Juergen's photo shows, some teeth can be rather large - his is 6 cm. As a rough rule one can use 1 inch of enamel = 6 feet of shark, making Juergens shark somewhere between 3.5 to 4 m long. The dark colour is because of the fossilization.
For an interesting account of origin and evolution of life in the sea, read: Aquagenesis. Richard Ellis, Penguin Book, 2001.
Sharks, silent hunters of the deep. Reader's Digest, 1986.
Guide to the sharks and rays of Southern Africa. LJV Compagno, DA Ebert, MJ Smale, 1989.