Description
As with most early trilobites, Mesonacis has an almost flat exoskeleton that is only thinly calcified, and has crescent-shaped eye ridges with a total length up to 2.8 inches in some fossils from the early cambrian. As part of the Olenellina suborder, Mesonacis lacks dorsal sutures. Like all other members of the Olenelloidea superfamily, 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 Olenellidae family that the frontal (L3) and middle pair (L2) of lateral lobes of the glabella are partially merged. This creates two very typical, isolated slits. The exoskeleton of Mesonacis is about 2⅓× as long as wide, measured between the genal angles. The outer ⅓ of the back (or posterior margin) of the headshield (or cephalon) angles forwards from the tip of the pleural spine to the genal angle. The central area of the cephalon (or glabella) and the frontal margin touch or the distance is as long as the margin at most (in jargon: the preglabellar field is short or absent). The thorax has approximately 25 segments, the pleura about 1½× as wide as the axis, excluding the genal spines. The 3rd segment carries extra large pleural spines (or macropleural spines) that reach back only to the tip of the 5th pleural spines. The segments look degenerated behind the 15th (or an opisthothorax can be distinguished). The tailshield (or pygidium) is very small and subquadrate in shape, and carries one or two pairs of small marginal spines.
Read more about this topic: Angustolenellus
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