These are fossils of trilobites; slater-like underwater creatures that lived in the oceans (including parts of Tasmania that were underwater!) when dinosaurs ruled the Earth. They make great ornamental gifts.
As these are real fossils, size does vary.
Largest pictured is 85 mm x 50 mm (approx)
Smallest pictures is 42 mm x 30 mm (approx)
Trilobites are picked at random, and may not be the ones pictured. As with all fossils, handle with care. Small parts, not suitable for young children.