Effacement, the loss of surface detail in the cephalon, pygidium, or the thoracic furrows, is also a common evolutionary trend.
42.
The former differs from the latter in having spines on both the cephalon and the pygidium, a characteristic unique to the entire genus.
43.
The thorax and pygidium are very regularly divided into up to 23 rather narrow segments, without a furrow within each of the pleurae.
44.
Its pygidium has a terminal anus, with two pygidial cirri being laterally inserted, as well as an unpaired appendage that is placed ventrally.
45.
In arachnids, the pygidium is formed by reduction of the last three opisthosomal segments to rings where there is no distinction between tergites and sternites.
46.
The pygidium is also very effaced, but the ovate pygidial axis ( or rhachis ) is well defined and a border furrow is also present.
47.
The Asaphids generally have cephalon ( head ) and pygidium ( tail ) parts similar in size, and most species have a prominent median ventral suture.
48.
Exigua is only known from the central part of the headshield or cranidium, so free cheeks ( or librigenae ), thorax and pygidium are unknown.
49.
The thorax is composed of eleven segments, with the relatively large pygidium with a slender axis of 11 to 16 rings and 6 or 7 pleural ribs.
50.
Its major characteristics are a wide cranidium, heavy eye ridges, longitudinal striae on the pre-glabellar area, and a small pygidium with few segments.
