Mesolenellus
Mesolenellus is an extinct genus of trilobites that lived during the lower Cambrian (Botomian), found in Greenland and Spitsbergen. Etymology. "Mesolenellus" is a contraction of meso - middle - and "Olenellus", the genus from which it was split off. The epithet "hyperborea" is a contraction of hyper (Greek ὑπέρ "excess") and Boreas (), god of the North Wind in Greek mythology, referencing the current geographical position of the deposits of its type location. "M. svalbardensis" has been named for the Norwegian island group Svalbard or Spitsbergen, where it was originally collected. Taxonomy. It was considered a subgenus of "Olenellus" before. The nearest  relatives of "Mesolenellus" are the species of the genus "Mesonacis", with which it constitutes the subfamily Mesonacinae. Habitat. The mudstone in which both species were found indicates deeper, quiet waters. Ecology. "Mesolenellus hyperborea" occurs together with "Limniphacos perspiculum", "Serrodiscus", hyoliths, "Petrianna fulmenta" (Bradoriida), and inarticulate brachiopods. Description. As with most early trilobites, "Mesolenellus" has an almost flat exoskeleton, that is only thinly calcified, and has crescent-shaped eye ridges. As part of the suborder Olenellina, "Olenellus" lacks dorsal sutures. 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. "Mesolenellus" also shares the typical character of the whole family Olenellidae that the frontal (L3) and middle pair (L2) of lateral lobes of the glabella are partially merged. This creates two very typical, isolated slits. "Mesolenellus" is a genus within the Mesonacinae, with eye-ridges curved but almost parallel to the midline. The back of the eye-ridges is opposite the most backward ring of the glabella (L0 or occipital ring). Genal spines are 6-8 times as long as L0. The outer furrows of the glabella are parallel to  the midline between the back of the cephalon and the furrow between side lobes L2 and L3. The thorax is 3 times as wide as the axis at the 3rd segment. "Mesolenellus" can be distinguished from the sister-genus "Mesonacis", where the back of the eye-ridge extends only to the most backward side lobes (L1), genal spines are only 1-5 times as long as L0, and the glabella widens forwards along L1 and L2. Except for in "Mesonacis fremonti" the curved eye-ridges are at an angle of 15°-20° with the midline. A third genus  Olenellus, that constitutes the second subfamily of the family Olenellidae differs from "Mesolenellus" in having genal spines 4-5 times as long as L0, the glabella widens forwards along L1 and L2, and the thorax 4-4½ times wider that the axis at the 3rd segment.