What is "bulbaspis"

Bulbaspis ("bulb shield") is a late Ordovician genus of asaphid trilobites of the family Raphiophoridae found primarily in upper Ordovician-aged deepwater marine strata of Kazakhstan, China, and possibly Tasmania. Species of Bulbaspis are similar to other raphiophorids such as Ampyx and Raphiophorus, save that the long spine that emanates from the glabella of the latter two genera has been modified into a knob-like or bulb-like structure in Bulbaspis that developed incrementally in the animal's growth. The function of the bulb is as yet unknown: one hypothesis suggests sexual selection may have had a role in its evolution in the genus.