Acastella sp.
Trilobite
Lower Silurian
Revesto Sbsvyte Formation
Kamyanets-Podolskiy Region, Kitajgorod, Ukraine.

25mm trilobite on 34mm matrix.

Acastella was a trilobite that lived during the Silurian and survived into the Devonian.  It is known from Europe and New Zealand but is uncommon wherever it occurs.