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1. Platelike.
a. Relating to or being a kind of fish scale that is a toothlike plate with a protruding spine, characteristic of sharks, skates, and rays.
b. Having placoid scales.

[Greek plax, plak-, flat stone, plate; see plāk- in Indo-European roots + -oid.]


1. (Biology) platelike or flattened
2. (Zoology) (of the scales of sharks and other elasmobranchs) toothlike; composed of dentine with an enamel tip and basal pulp cavity
[C19: from Greek plac-, plax flat]


(ˈplæk ɔɪd)

platelike, as the scales or dermal investments of sharks.
[1835–45; < Greek plak- (s. of pláx) something flat, tablet]
ThesaurusAntonymsRelated WordsSynonymsLegend:
Adj.1.placoid - as the hard flattened scales of e.g. sharks
planar, two-dimensional - involving two dimensions
References in periodicals archive ?
All are cartilaginous, have five to seven pairs of gills, rigid dorsal fins and small, hard, tooth-like placoid scales that protect their exterior surface.
The family is characterised by the absence of trichobothria on the antennae; the presence of mostly symmetrical, unsegmented chitinous hooks at tip of abdomen; a maximum of four placoid sensilla on terminal article of antennae; thorax with two pairs of spiracles on sternites II and III; abdominal sternite 1 with a pair of styles and a pair of lateral subcoxal organs.
Scutellum a little wider than long, almost as long as mesoscutum; scutellar placoid sensilla in the middle of scutellum and close to each other.
These placoid scales have a drop-like, inclined, elongate crown, well-developed neck and small base with concave basal surface (Fig.
We considered placoid chorioretinitis to be the presence of [greater than or equal to] 1 placoid, yellowish, outer retinal lesions.
Acute posterior multifocal placoid pigment epitheliopathy (APMPEE) is a chorioretinal inflammatory disease of unknown origin.
Ocular manifestations seen in HIV-associated neurosyphilis include posterior uveitis, panuveitis, posterior placoid chorioretinitis [5], retinal vasculitis, and, on rare occasions, bilateral optic neuritis [3,6].
The differential diagnosis includes Stargardt disease, cone dystrophy, enhanced S-cone syndrome, pericentral retinitis pigmentosa, syphilitic placoid chorioretinitis, acute zonal occult outer retinopathy (AZOOR), posterior scleritis, traumatic retinopathy (commotio retinae), posterior uveitis, hydroxychloroquine toxicity, and autoimmune retinopathy [18-22].
Metastases are typically placoid shaped with an undulating surface and show medium to high blood flow velocity.
The disease is characterized by infectious etiology and overlapping clinical features in an intermediary form of acute posterior multifocal placoid pigment epitheliopathy and serpiginous choroiditis.
Acute, posterior, multifocal, placoid, pigment epitheliopathy and Lyme disease.
Shark skins bear thousands of rigid denticles, or placoid scales, comprised of hard materials, such as dentin and enamel, which are high in strength and stiffness (Raschi and Tabit, 1992).