What is "mesomyzon"

Mesomyzon mengae is an extinct lamprey from freshwater strata of the Early Cretaceous-aged Yixian Formation, in China. So far, M. mengae is the only lamprey, and by default, the only agnathan known from post-Paleozoic fossils.

The animal's exquisitely preserved fossils show a creature very similar to modern-day lampreys, having a well-developed sucking oral disk, a branchial basket, at least seven pairs of gill pouches and corresponding gill arches, impressions of gill filaments, and at least 80 myomeres of its musculature.