The placoid scales on a shark are a phalanx of precise dermal teeth.    From Wordnik.com.  [A World Ready for Its Close-Up] Reference

Steenstrup -- had been struck with the resemblance existing between the placoid scales and the teeth of Elasmobranch fishes.    From Wordnik.com.  [Form and Function A Contribution to the History of Animal Morphology] Reference

The new species is highly adapted to hypogean life with very obvious troglobiomorphic features: unpigmented cuticle, an extraordinary lengthening of thorax and appendixes, multiplication of antennomeres and supernumerary placoid sensilla, not just in the apical antennomere but also in the preceding antennomeres.    From Wordnik.com.  [Archive 2007-01-01] Reference

Finally, with respect to the ‘Vertebrata’, the same law holds good: certain types, such as those of the ganoid and placoid fishes, having persisted from the palaeozoic epoch to the present time without a greater amount of deviation from the normal standard than that which is seen within the limits of the group as it now exists.    From Wordnik.com.  [Essays] Reference

Hertwig followed up this clue, and came to the conclusion not only that placoid scales and teeth were strictly homologous, but also that all membrane bones were derived phylogenetically from ossifications present in the skin or in the mucous membrane of the mouth, just as cartilage bones were derived from the cartilaginous skeletons of the primitive Vertebrates.    From Wordnik.com.  [Form and Function A Contribution to the History of Animal Morphology] Reference

The first Vertebrata to appear in the fossil history of the world are fishes; fish spines and placoid scales.    From Wordnik.com.  [Text Book of Biology, Part 1: Vertebrata] Reference

Cycloid Fish Scale - The primary types of fish scales include placoid, cosmoid, ganoid, ctenoid, and cycloid scales.    From Wordnik.com. Reference

How placoid scales may have given rise to these structures will be understood by considering such a bone as the vomer of the frog.    From Wordnik.com.  [Text Book of Biology, Part 1: Vertebrata] Reference

Besides the teeth there perhaps remain relics of the placoid scales in the anatomy of the higher vertebrata, in the membrane bones.    From Wordnik.com.  [Text Book of Biology, Part 1: Vertebrata] Reference

But one of the most remarkable weapons of the period was the sting of the Pleuracanthus, another great placoid of the age of gigantic fishes.    From Wordnik.com.  [The Testimony of the Rocks or, Geology in Its Bearings on the Two Theologies, Natural and Revealed] Reference

Dogfish Shark placoid Scales - The bodies of dogfishes, like all sharks, are externally lined with placoid scales, also known as dermal denticles.    From Wordnik.com. Reference

The skin of the dog-fish is closely set with pointed tooth-like scales, the placoid scales, and these are continued over the lips into the mouth as teeth.    From Wordnik.com.  [Text Book of Biology, Part 1: Vertebrata] Reference

Old Red Sandstone, -- some were remarkable for the strangeness of their forms, and some for constituting links of connection which no longer exist in nature, between the ganoid and placoid orders.    From Wordnik.com.  [The Testimony of the Rocks or, Geology in Its Bearings on the Two Theologies, Natural and Revealed] Reference

This bone lies on the roof of the frog's mouth, and bears a number of denticles, and altogether there is a very strong resemblance in it to a number of placoid scales the bony bases of which have become confluent.    From Wordnik.com.  [Text Book of Biology, Part 1: Vertebrata] Reference

These facts seem to point to stages in the fusion of placoid bases, and their withdrawal from the surface to become incorporated with the cranial apparatus as membrane bones, a process entirely completed in the mammalian type.    From Wordnik.com.  [Text Book of Biology, Part 1: Vertebrata] Reference

Finally, with respect to the 'Vertebrata', the same law holds good: certain types, such as those of the ganoid and placoid fishes, having persisted from the palaeozoic epoch to the present time without a greater amount of deviation from the normal standard than that which is seen within the limits of the group as it now exists.    From Wordnik.com.  [Lectures and Essays] Reference