Mesonacis is an extinct genus of trilobite that lived during the Botomian, found in North-America (excluding Greenland), and the United Kingdom (North-Western Scotland).

[2] Mesonacis vermontanus occurs in the middle Upper Olenellus-zone of Vermont (Parker Slate, Georgia).

[1] Mesonacis eagerensis is present in the Olenellus-zone of British Columbia, Canada (Eager Formation, just South of the Fort Steel-St. Eugene Mission road, 6 miles North-East of Cranbrook).

Like all other members of the superfamily Olenelloidea, the eye-ridges spring from the back of the frontal lobe (L4) of the central area of the cephalon, that is called glabella.

Mesonacis also shares the typical character of whole family Olenellidae in that the frontal (L3) and middle pair (L2) of lateral lobes of the glabella are partially merged.