Radotina and other tesserate fishes

It is shown that the stratigraphical assignment of Radotina needs considerable revision. Radotina tuberculata Gross is conspecific with Coccosteus primus Barrande and should be known as R. prima (Barrande); this species differs from its contemporary R. tesselata in the same way as extreme forms of R. kosorensis but the question of further synonymy is unsettled. The dermal bones of Radotina are considered to represent a, modified pattern closely related to that of Macropetaliehthyida; to this general grouping may be added the Rhenanida and Stensioellida. It is highly probable that the Radotinida and Acanthothoraci are synonymous.
Consideration of the tesserate condition in arthrodires suggests that this is a secondary condition in the known members. The Psammosteida also show the development of secondary tesseration among the Heterostraci. An hypothesis based on Holmgren's principle of delamination is apparently illustrated by these forms. In certain Rhenanida this delamination goes so far as to produce cartilage bones in the same position, but at a deeper level, as compared with dermal bones in earlier forms.