Cephalaspidomorphi

Cephalaspidomorphi (alternatively called Monirhina, or simply cephalaspids) is a class of jawless fishes that is presently regarded as uniting the osteostracans, galeaspids and pituriaspids. Most biologists regard this taxon as extinct, but the name is still sometimes used in the classification of lampreys because they were once thought to be descended. If lampreys are included they would extend the known range of the group from the Silurian and Devonian periods, when they are traditionally assumed to have lived, to the present day. Modern works typically assume the cephalaspidimorphs to be the closest relatives of jawed fishes, who may have emerged from within them; if this is true, they would survive if the jawed fish are included. The cephalaspidomorphs possessed armored head-shields, a heterocercal tail fin, and in some groups paired pectoral fins.

==Biology and morphology== thumb|254px|left|Reconstruction of Cephalaspis lyelli Cephalaspidomorphs possessed armored plates on the exterior of their bodies. The head shield was particularly well developed, protecting the head, gills and the anterior section of the viscera. The body was in most forms well armored as well. The head shield had a series of grooves over the whole surface, forming an extensive lateral line organ. The eyes were rather small and placed on the top of the head. There was no jaw, and instead the mouth opening was surrounded by small plates, making the lips flexible, but without any ability to bite.