Online Encyclopedia

Search over 40,000 articles from the original, classic Encyclopedia Britannica, 11th Edition.

OSTRACODERMS

Online Encyclopedia
Originally appearing in Volume V20, Page 361 of the 1911 Encyclopedia Britannica.
» Make a correction to this article.
» Add information or comments to this article.
Spread the word: del.icio.us del.icio.us it!

OSTRACODERMS or OSTRACOPHORES, the earliest and most See also:

primitive See also:group of See also:fish-like animals, found as fossils in Upper From the Trans. See also:Roy. See also:Soc., See also:Edinburgh. From the Proc. Geol. Assoc. See also:Silurian and Devonian formations both in See also:Europe and in See also:North See also:America. They are so named tGr. See also:shell-skins or shell-bearers) paired fins. They must also have been provided with the usual in allusion to the nacreous shell-like See also:appearance of the inner gill-apparatus, but there is See also:reason to believe that their See also:lower See also:face of the plates of See also:armour which See also:cover the more See also:common See also:jaw was not on the fish See also:plan. They are, therefore, at least as See also:low in the zoological See also:scale as the existing lampreys, with which See also:Cope, See also:Smith, See also:Woodward and others have associated them. They are all small animals, many of them only a few centimetres in length. The See also:oldest and lowest See also:family of Ostracoderms, that of Coelolepidae, is known by nearly See also:complete skeletons of Thelodus (fig.

I) and Lanarkia from the Upper Silurian mudstones of See also:

Lanarkshire, See also:Scotland. The See also:body is completely and uniformly covered with See also:minute granules which resemble the See also:shagreen of sharks, and were erroneously ascribed to sharks when they were first discovered in the Upper Silurian See also:bone-See also:bed at See also:Ludlow, See also:Shropshire. The See also:head and anterior See also:part of the See also:trunk are depressed and shown from above or below in the fossils, and this region sharply contracts behind into the slender tail, which is generally seen in See also:side view, with one small dorsal fin and a forked heterocercal tail. The eyes are far forwards and wide apart. In another family, that of the Cephalaspidae (fig. 2), the animals resemble the Coelolepids in shape, but their skin-granules are fused into small plates, which are polygonal where there must have been much flexibility, and in rings See also:round the tail where the underlying successive plates of muscle necessitated this arrangement. The eyes are See also:close together. At the opening of the gill-cavity on each side at the back of the head, there is a flexible flap, which is sometimes interpreted as a paired See also:limb. Part of the armour of the Cephalaspidians contains bone-cells, but the dermal plates of two other families, the Pteraspidae (fig. 3) and Drepanaspidae, consist merely of fused shagreen granules without any advance towards bone. The Pteraspidae are interesting as showing on the inner side of the dorsal See also:shield impressions which suggest that the gill-cavities extended unusually far forwards to the front of the head. Another family, known only by nearly complete skeletons from the Upper Silurian mudstones of Lanarkshire, is that of the Birkeniidae, comprising small fusiform See also:species which are covered with granules disposed in curiously-arranged rows.

The highest Ostracoderms are the Asterolepidae, which occur only in Devonian rocks and include the See also:

familiar Pterichthys (fig. 4) from the See also:Middle Old Red See also:Sandstone of Scotland. In this family the primitive skin-tubercles seem to have fused, not into polygonal plates, but along the Iines of the slime-canals. The Asterolepid armour consists of symmetrically arranged, overlapping plates on the See also:top of the head and round the body, with a pair of flippers similarly armoured and appended to the latter. The tail resembles that of other Ostracoderms and is sometimes covered with scales. See E. See also:Ray Lankester, The Cephalaspidae (Monogr. Palaeont. Soc. 1868, 1870); R. H. See also:Traquair, The Asterolepidae (Monogr.

Palaeont. Soc. 1894, 1904, igo6) and papers in Trans. Roy. Soc. Edinb. vol. xxxix. No. 32 0899), vol. xl. Nos. 30, 33 (1903, 1905) ; A. S. Woodward, Catal.

See also:

Foss. Fishes, B.M. pt. ii. (1891); W. H. See also:Gaskell, Origin of Vertebrates (See also:London, 1908). (A. S.

End of Article: OSTRACODERMS

Additional information and Comments

There are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML.
Site content, images, and layout Copyright © 2006 - Net Industries, worldwide.
Do not copy, download, transfer, or otherwise replicate the site content in whole or in part.

Links to articles and home page are always encouraged.

[back]
OSTRACISM 		[next]
OSTRAU

Site © 2006 - Net Industries