The Chondrostei includes the chondrosteids, acipenserids and polyodontids. Although the characteristics are difficult to list formally, the group is probably homogenous, in the sense that its members are of probable common ancestry. However, the problem of the Chondrostei is that the boundary between it and later groups cannot be defined with certainty. Therefore, the Chondrostei are best defined as belonging to a primitive radiation of fishes having reached a certain evolutionary grade.