Trinucleus
the type genus of Tri nucleida. Body distinctly trilobed; margin of head-shield composed of two lamellæ, and perforated by numerous formamina; genal angles prolonged into conspicuous spines, usually single, but forked in T.
T. pongerardi; glabella prominent and pear-shaped with mere traces of lateral grooves; facial sutures rudimentary; cheeks tumid, and generally furnished on each side with a small tubercle seemingly representing the eyes; body rings six; tail triangular, with a distinct axis, and having its margin entire and striated.