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.
Lizards are reptiles which have a transverse external anal opening (instead of a longitudinal slit as in Crocodilians and tortoises) and which have the right and left halves of the mandibles connected by a sutural symphysis.
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.
- No shell-eyes: sutural laminae separated; slits in the valves 1 -7 do not correspond with the ribs of the tegmentum.
With shell-eyes and united sutural laminae.
Vermiform, with thick girdle and small valves; insertion and sutural plates strongly drawn forward, sharp and smooth.
Between the eyes and the tail-piece in all the orders nineteen segments are counted, the proof of a segment's existence depending on its separateness, complete or partial, or on a sutural indication, or else on the pair of appendages known to belong to it.
The carapace, formerly referred only to the antennar-mandibular segments, may perhaps in fact contain elements from any number of other segments of head and trunk, Huxley, Alcock, Bouvier giving support to this opinion by the sutural or other divisional lines in Potamobius, Nephrops, Thalassina, and various fossil genera.