How are paleozoic placoid scales of shark different from paleoniscoid scales of modern sharks?

Placoid Scale of paleozoic sharks - had pulp cavity, basal canal and neck canal. Also had evolved and an enameloid layer covered the dentine.

Paleoniscoid scales of modern shark - have lost the layer of vascular bone but retained the dentinous layer and laminated bony layer. Also have thick ganoid layyer.