Galeaspids are known from the Silurian and Devonian of China and Vietnam. They are characterized by a large, median dorsal inhalent opening and a scallopped pattern of the sensory-lines. Their mouth and gill openings are situated on the ventral surface of the head (top right). In the most primitive forms, such as the Silurian genus Hanyangaspis (top), the median dorsal inhalent opening is broad and situated anteriorly. In other galeaspids, its is more posterior in position and can be oval, rounded, heart-shaped or slit-shaped. In some Devonian galeaspids, such as the hunanaspidiforms Lungmenshanaspis (middle) and Sanchaspis (bottom right), the headshield is produced laterally and anteriorly into slender processes. The eugaleaspidiforms, such as Eugaleaspis (bottom left) have a horseshoe-shaped headshield and a slit-shaped median dorsal opening, which mimates the aspect of the headshield of osteostracans.