from The American Heritage® Dictionary of the English Language, 5th Edition.
- adjective Platelike.
- adjective Relating to or being a kind of fish scale that is a toothlike plate with a protruding spine, characteristic of sharks, skates, and rays.
- adjective Having placoid scales.
from The Century Dictionary.
- Plate-like: noting the dermal investments of sharks, which take the place of true scales and are the ossified papillæ of the cutis.
- Having placoid scales, as a fish; belonging to the Placoidei. See cut under
- noun A member of the Placoidei.
from the GNU version of the Collaborative International Dictionary of English.
- noun Any fish having placoid scales, as the sharks.
- noun One of the Placoides.
- adjective (Zoöl.) Platelike; having irregular, platelike, bony scales, often bearing spines; pertaining to the placoids.
from Wiktionary, Creative Commons Attribution/Share-Alike License.
platelike; having irregular, platelike, bony scales, often bearing spines; pertaining to the placoids.
- noun Any fish having placoid scales, such as the sharks.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- adjective as the hard flattened scales of e.g. sharks
from The American Heritage® Dictionary of the English Language, 4th Edition
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.
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.
The placoid scales on a shark are a phalanx of precise dermal teeth.
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.
Steenstrup -- had been struck with the resemblance existing between the placoid scales and the teeth of Elasmobranch fishes.
How placoid scales may have given rise to these structures will be understood by considering such a bone as the vomer of the frog.
The first Vertebrata to appear in the fossil history of the world are fishes; fish spines and placoid scales
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.
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.
Besides the teeth there perhaps remain relics of the placoid scales in the anatomy of the higher vertebrata, in the membrane bones.