| 释义 |
placoid, a. and n. Zool.|ˈplækɔɪd|
[f. Gr. πλάξ, πλακ- flat plate, tablet: see -oid. Cf. F. Placoides, in mod.L. form Placoidei, name given by Agassiz, 1833, to certain fishes, on account of the plate-like appearance of their scales. (The earliest derivative in Eng. was app. placoidean.)]
A. adj.
1. Having the form of a plate; applied to the horny scales and tubercles of the Placoidei: see B.
1842H. Miller O.R. Sandst. 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. 1870Rolleston Anim. Life Introd. 68 The dermal exo-skeleton may take the form of..placoid or spiny dentinal formations. 1880Günther Fishes 349 Very young individuals possess a series of small ‘placoid’ spines. 1888Rolleston & Jackson Anim. Life 411 The primitive form [of the exoskeleton] occurs..in the shape of dermal teeth (= placoid scales), similar in structure and development to oral teeth.
2. Having placoid scales; of or pertaining to the Placoidei: see B.
1847Nat. Encycl. I. 136 A genus of fossil Placoid fishes. 1851Richardson 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. 1880Günther Fishes 21 The distinctions between..placoid and ganoid fishes are vague.
B. n. A fish of the division Placoidei, containing the sharks and rays, distinguished by having the skin protected by irregularly disposed bony scales, sometimes bearing spines.
1854H. Miller Sch. & Schm. xxi. (1858) 473 The mere detached teeth and spines of placoids. 1873Dawson Earth & Man v. 96 The Placoids or shark-like fishes.
Hence plaˈcoidal a. rare; plaˈcoidean a. and n.
1836Buckland Geol. & Min. I. xiii. 269 note, Placoidians... Fishes of this Order are characterized by having their skin covered irregularly with plates of enamel [etc.]. Ibid. 283 Genera of the first and second orders (Placoidean and Ganoidian)..ceased suddenly. 1845R. Chambers Vestiges (ed. 4) 207 When fishes came, the first forms were those ganoidal and placoidal types which correspond with the early fœtal condition of higher orders. 1849Smart Dict. Suppl., Placoideans. |