The type species of the genus. It is a thin-plated species from the Eifelian-aged Aruküla beds of Estonia. Although H. sulcatus was described earlier in 1837, H. formosissimus is the official type species as it was the first to be described as a placoderm (H. sulcatus was originally described as a soft-shelled turtle). H. formosissimus had a small, thin keel down the dorsal-center of its median dorsal plate.