Galeaspida
Galeaspida (from Latin, 'Helmet shields') is an extinct taxon of jawless marine and freshwater fish. The name is derived from "galea", the Latin word for "helmet", and refers to their massive bone shield on the head. Galeaspida lived in shallow, fresh water and marine environments during the Silurian and Devonian times (430 to 370 million years ago) in what is now Southern China, Tibet and Vietnam. Superficially, their morphology appears more similar to that of Heterostraci than Osteostraci, there being currently no evidence that the galeaspids had paired fins. However, Galeaspida are in fact regarded as being more closely  related to Osteostraci, based on the closer similarity of the morphology of the braincase. Morphology. The defining characteristic of all galeaspids was a large opening on the dorsal surface of the head shield, which was connected to the pharynx and gill chamber, and a scalloped pattern of the sensory-lines. The opening appears to have served both the olfaction and the intake of the respiratory water similar to the nasopharyngeal duct of hagfishes. Galeaspids are also the vertebrates which have the largest number of gills, as some species of the order Polybranchiaspidida (literally "many gills shields") had up to 45  gill openings. The body is covered with minute scales arranged in oblique rows and there is no other fin besides the caudal fin. The mouth and gill openings are situated on the ventral side of the head, which is flat or flattened and suggests that they were bottom-dwellers. Taxonomy. There are around 76 + described species of galeaspids in at least 53 genera. If the families Hanyangaspidae and Xiushuiaspidae can be ignored as basal galeaspids, the rest of Galeaspida can be sorted into two main groups: the first being the order Eugaleaspidiformes, which comprises the genera "Sinogaleaspis", "Meishanaspis", and  Anjianspis, and the family Eugaleaspididae, and the second being the Supraorder Polybranchiaspidida, which comprises the order Polybranchiaspidiformes, which is the sister taxon of the family Zhaotongaspididae and the order Huananaspidiformes, and the family Geraspididae, which is the sister taxon of Polybranchiaspidiformes + Zhaotongaspididae + Huananaspidiformes. Some experts demote Galeaspida to the rank of subclass, and unite it with Pituriaspida and Osteostraci to form the class Monorhina. Fossil Record. The oldest known galeaspids, such as those of the genera "Hanyangaspis" and "Dayongaspis", first appear near the start of the Telychian age, of the latter half of the Llandovery epoch of the  Silurian, about 436 million years ago. During the transition from the Llandovery to the Wenlock, the Eugaleaspids underwent a diversification event. By the time the Wenlock epoch transitioned into the Ludlow Epoch, all of the eugaleaspids, save for the Eugaleaspidae, were extinct. The Eugaleaspidae lived from the Wenlock, and were fairly long-lived, especially the genus "Eugaleaspis". The last of the Eugaleaspididae disappeared by the end of the Pragian Epoch of the Lower Devonian. The first genus of Geraspididae, the eponymous "Geraspis", appears during the middle of the Telychian. The other genera of Polybranchiaspidida appear in the fossil record a  little after the beginning of the Lochkovian Epoch, at the start of the Devonian. The vast majority of the supraorder's genera either date from the Pragian epoch, or have their ranges end there. By the time the Emsian epoch starts, only a few genera, such as "Duyunolepis" and "Wumengshanaspis", survive, with most others already extinct. The last galeaspid is an as yet undescribed species and genus from the Fammenian epoch of the Late Devonian, found in association with the tetrapod "Sinostega" and the antiarch placoderm "Remigolepis", in strata from the Northern Chinese province of Ningxia.