Bumastus, genus of trilobites (extinct arthropods) found in Europe and North America as fossils in rocks of Ordovician to Silurian age (between 408 and 505 million years old). Bumastus is very distinctive in form; the head and tail regions are smooth and very large and have fused segments. Its elliptical body is flat on the bottom; the back and sides are strongly concave and arched. Bumastus was a bottom-dwelling form.