Procambarus primaevus
Crustacean
Early Eocene
Green River Formation
Kemmerer, Wyoming, USA

62mm freshwater crayfish on 68mm slab.

Procambarus is a freshwater crayfish and P. primaevus is actually one of the first invertebrates ever described from the Green River sandwich beds.  The genus still lives today in North and Central America, preferring warm, slow-moving river, stream, or marsh habitats.