The most primitive form of the first type of dermal structures is that of the placoid scales of Elasmobranchii.
The Thornback Skate is covered with prickles (placoid scales).
The teeth of Selachii are fundamentally identical with placoid scales.
In the groups of fishes above the Selachians, the coating of placoid elements shows various modifications.
In the Selachians the bony skeleton is restricted to the placoid elements.
In some cases the placoid scales are enlarged to form plates or spines capped or coated with enamel.