1) Animal order 2) Cephalaspida 3) Suborder cephalaspida 4) Suborder osteostraci 1) Cephalaspida 
(from the article `agnathan`) ...lampreys are adaptations to a burrowing habit—throughout life in the hagfish, during the larval period in the lamprey. Considerable variation in ... ...of fossil groups uncertain; earliest probably in fresh water.Subclass MonorhinaWith 1 nostril.†Order OsteostraciLate Silurian to close of ... ...
[n] - extinct group of armored fishlike vertebrates
suborder Osteostraci noun extinct group of armored fishlike vertebrates; taxonomy is not clear