Like other trilobites, Brongniartella is composed of three parts: a cephalon, a multisegmented thorax, and a pygidium.[1]
The cephalon is semicircular or semielliptical and lacks a border. The glabella is well-defined and urn-shaped or tapering forward, and generally lacks furrows. The axial furrows strongly converge in front of the occipital ring. The facial sutures are gonatoparian as they intersect the margin of the cephalon near the genal angles. The rostral suture is a wide curve on the upper side of the cephalon that is near or on the anterior margin.[1]
The 13-segmented thorax has a wide but indistinct axis, with extremely shallow axial furrows and the axis and pleurae being almost undistinguishable from the general concavity of the body.[2][1][3] The axial furrows of the thorax form a continuous depression, but each axial ring is also independently separated from the corresponding pleura by a shallow, transverse, oblique furrow corresponding to the one crossing the occipital ring from the base of the glabella. The base of each pleura between this transverse furrow and the pleural furrow is slightly swollen, and there is a peg-like interior projection on the posterior margin of each thoracic segment that fits into a corresponding notch of the succeeding thoracic segment.[3]
The pygidium is rounded and parabolic with a well-defined axis that ends in front of the posterior margin and a slightly indistinct, narrow posterior border. The pygidial ribs do not extend to the lateral margin of the pygidium. It is composed of 9-12 segments, and some forms bear a postaxial ridge.
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