trinucleus

from The Century Dictionary.

• noun A genus of small blind hypoparian trilobites, typical of the family Trinucleidæ, with relatively large creseentic cephalic shield provided with prominent tumid glabella and cheeks, broad pitted border, and long sharp genal spines, thorax with six segments, and a small pygidium. The genus is indicial of Lower Silurian formations.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Paleon.) A genus of Lower Silurian trilobites in which the glabella and cheeks form three rounded elevations on the head.