placoidadj.n.
/ˈplakɔɪd/, U.S. /ˈplæˌkɔɪd/ A. adj.
1. Zoology.
a. Designating a type of fish scale resembling a small tooth with a broad flat base, made of dentine with a pointed backward projection of enamel, as forming the dermal armour of cartilaginous fishes. Cf. ctenoid adj. 1, ganoid adj. 1.
ΘΚΠ
the world > animals > fish > parts of fish > [adjective] > having scales > of scales of specific type
fulcral1809
placoid1842
cycloid1847
ctenoid1872
cycloidal1872
1842 H. Miller Old Red Sandstone (ed. 2) iv. 73 One kind of scale, for instance the Placoid or broad plated scale, is found to characterize all the cartilaginous fishes of Cuvier except the sturgeon.
1870 G. Rolleston Forms Animal Life (ed. 2) Introd. 68 The dermal exo-skeleton may take the form of..placoid or spiny dentinal formations.
a1933 J. A. Thomson Biol. for Everyman (1934) I. xvii. 439 The Selachians are marked by having a cartilaginous endoskeleton, tooth-like placoid scales.., and no air-bladder.
1968 Van Nostrand's Sci. Encycl. (ed. 4) 1258 Class Chondrichthyes (Elasmobranchii). Shark-like fishes with cartilaginous skeleton; exoskeleton of placoid denticles.
2000 C. Tudge Variety of Life ii. xiv. 356 In sharks, the formidable teeth are like a continuation of the placoid scales that cover the whole body.
b. Having or characterized by placoid scales; spec. of or relating to a former order Placoidei of cartilaginous fishes which included sharks and rays. Now rare.
ΘΚΠ
the world > animals > fish > subclass Elasmobranchii > [adjective] > of or relating to Placoidei
placoidean1836
placoid1842
placoidal1845
1842 Geologist 1 57 Rays, belonging to Professor Agassiz's placoid order.
1851 G. F. Richardson Introd. Geol. (1855) 275 The first order, or Placoid,..have the skin irregularly covered with plates of enamel, sometimes large, as in the rays, sometimes reduced to small points, as in the sharks.
1880 A. Günther Introd. Study of Fishes 21 The distinctions between..placoid and ganoid fishes are vague.
2. Biology. Resembling a plate in shape; flattened; esp. designating a type of sensillum in insects.
ΚΠ
1942 A. D. Imms Outl. Entomol. 35 Olfactory receptors..are of various types..and may be placoid (hive bee), basiconic (butterflies and Muscid flies), trichoid (Rhodnius and other insects), &c.
1968 Van Nostrand's Sci. Encycl. (ed. 4) 1480 Placoid sensillae end in a thin porous plate or membrane covering a canal.
1987 Proc. Royal Soc. B. 232 345 No cuticular pits or openings were identified on the tarsi that could have indicated placoid or coeloconical sensilla.
1995 Ophthalmology 102 790 (heading) Choroidal hypoperfusion in acute posterior multifocal placoid pigment epitheliopathy.
B. n.
1. Zoology. = placoidean n.
ΘΚΠ
the world > animals > fish > subclass Elasmobranchii > [noun] > member of division Placoidei
placoidean1836
placoid1845
1845 R. I. Murchison in Q. Jrnl. Geol. Soc. 1 484 Among the Cestraciont family of Placoids he [sc. Agassiz] places two of the Russian forms.
1852 D. T. Ansted Physical Geogr. xii, in Man. Geogr. Sci. I. 381 The order of Placoids, divided into seven families, is represented in a fossil state by genera referred to every family but one (Cyclostoma).
1873 J. W. Dawson Story Earth & Man v. 96 The Placoids or shark-like fishes.
1935 Amer. Midland Naturalist 16 833 The armor of a ‘placoid’ (Dinichthys) [has] been found near Rock Island, Illinois.
1960 E. Lurie Louis Agassiz: Life in Sci. iii. 81 His [sc. Agassiz's] taxonomy, a division of ancient fishes into orders he called Cycloids, Ctenoids, Ganoids, and Placoids, was based upon an effort to understand the distinctions among fossil forms by inferences drawn from his knowledge of living fishes.
2. Biology. Any of various flattened, plate-like structures.
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1874 Proc. Royal Soc. 1873–4 22 521 (note) To flat cells [of connective tissue] the term placoids has been applied by Dr. Burdon Sanderson, the equivalent of the German platten.
1883 Proc. Royal Soc. 1882–3 34 157 The teeth and placoids of the Plagiostomi.
1965 Systematic Zool. 14 25/2 In all branchiobdellids the anterior part of the pharynx..is furnished with dorsal and ventral placoids (jaws) composed of a hardened secretion, presumably sclerotized chitin.
1973 Amer. Jrnl. Bot. 60 633/2 The lamellate portions of intravacuolar reticulum represent small cytoplasmic placoids, multidirectionally extending to form filaments.
1996 Systematic Biol. 45 205 Although heterotardigrades and eutardigrades differ in many respects, they share..many characters (similar stylet apparatus, fixed number of appendages, pharyngeal bulb with placoids, etc.).
Derivatives
plaˈcoidal adj. rare = sense A. 1a.
ΘΚΠ
the world > animals > fish > subclass Elasmobranchii > [adjective] > of or relating to Placoidei
placoidean1836
placoid1842
placoidal1845
1845 R. Chambers Vestiges Nat. Hist. Creation (ed. 3) 207 When fishes came, the first forms were those ganoidal and placoidal types which correspond with the early fœtal condition of higher orders.
1940 Q. Rev. Biol. 15 75/2 Differences in the origin and development and distribution of mantle dentin and circumpulpar dentin are shown for the placoidal organs and the various types of teeth of several species of sharks and rays.
This entry has been updated (OED Third Edition, June 2006; most recently modified version published online June 2022).
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adj.n.1842