American Heritage® Dictionary of the English Language, Fourth Edition
- adj. Platelike, as the hard toothlike scales of sharks, skates, and rays.
Century Dictionary and Cyclopedia
- Plate-like: noting the dermal investments of sharks, which take the place of true scales and are the ossified papillæ of the cutis. In combination they form the shagreen of the sharks. The name is also extended to the tubercular or thorn-like armature of the skin in rays.
- Having placoid scales, as a fish; belonging to the Placoidei. See cut under scale
- n. A member of the Placoidei.
- adj. platelike; having irregular, platelike, bony scales, often bearing spines; pertaining to the placoids.
- n. Any fish having placoid scales, such as the sharks.
GNU Webster's 1913
- adj. (Zoöl.) Platelike; having irregular, platelike, bony scales, often bearing spines; pertaining to the placoids.
- n. Any fish having placoid scales, as the sharks.
- n. One of the Placoides.
- adj. as the hard flattened scales of e.g. sharks
- Greek plax, plak-, flat stone, plate; see plāk-1 in Indo-European roots + -oid. (American Heritage® Dictionary of the English Language, Fourth 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.”
“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.”
“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.”
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