Along the sutural border of the elytron, the chitinous lamella forms a tubular space within which are numerous glands.
But avicularia or vibracula may also occur in other places - on the backs of unilaminar erect forms, along the sutural lines of the zooecia and on their frontal surface.
In this genus the number of segments composing the pygidium is obscured, as also it is in the genus Illaenus, which has as many as ten movable segments preceding the large semi-circular pygidium; but in such forms as Ogygia and Asaphus, which have about eight free segments, the sutural lines on the pygidium indicate that it is composed of about a dozen or more segments.
The anterior margin of each valve except the first is provided with two projections called sutural laminae which underlie the posterior margin of the preceding valve.
Vermiform, with thick girdle and small valves; insertion and sutural plates strongly drawn forward, sharp and smooth.
