The class Osteostraci is an extinct taxon of bony-armored jawless fish, termed "ostracoderms", that lived in what is now North America, Europe and Russia from the Middle Silurian to Late Devonian.
Osteostraci
Parameteoraspis fossilized headshield
Agnatha is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both living (cyclostomes) and extinct species. Among recent animals, cyclostomes are sister to all vertebrates with jaws, known as gnathostomes.
Agnatha
Image: Eptatretus hexatrema 15727711
Image: Pacific lamprey facing right
Image: Thyestes verrucosus 22DB