GALEASPIDA
Rather like the Osteostraci, the Galeaspida are armoured jawless vertebrates with a large elongated head-shields. In the anterior portion of the head-shield they have a large median dorsal opening and the dorsal surface is decorated by the main and infraorbital lateral-line canals. Most Galeaspids seemed to be benthic forms, living in sandy or muddy marginal, marine environments
Polybranchiaspis is common in the Lower Devonian of Southern China and Vietnam. It had an oval head-shield where the ornamentation consists of tiny, closely set, fused tubercles arranged in radiating rows. The median dorsal opening is beanshaped with the eyes dorsal in position. The exoskeleton is hollowed by large, polygonal cavities that are similar to the honeycomb-like structure of the heterostraci (although probably not homologous). Another osteostracan feature is the vascular network and sensory canals between the exo and endoskeleton
Polybranchiaspis liaojaoensis, Cuifengshan
Formation, Yunnan,