. The Americana : a universal reference library, comprising the arts and sciences, literature, history, biography, geography, commerce, etc. of the world. nes of many speciesof sharks and chimaeroids, fishes otherwise un-known, occur in the rocks. These are calledichthyodoroulites, and their proper classificationis often a matter of much uncertainty. Theearliest of these are known as Onchus, occur-ring in the Upper Silurian. Class Ostracophori.— The earliest verte-brates actually recognized as fossils are knownas ostracophores CfoTpaKov, a box; <popiw, tobear). These are most extraordinary
. The Americana : a universal reference library, comprising the arts and sciences, literature, history, biography, geography, commerce, etc. of the world. nes of many speciesof sharks and chimaeroids, fishes otherwise un-known, occur in the rocks. These are calledichthyodoroulites, and their proper classificationis often a matter of much uncertainty. Theearliest of these are known as Onchus, occur-ring in the Upper Silurian. Class Ostracophori.— The earliest verte-brates actually recognized as fossils are knownas ostracophores CfoTpaKov, a box; <popiw, tobear). These are most extraordinary creatures,which may be described as jawless, limbless,enveloped in a coat of mail. While they havebeen called mailed lampreys, the likeness tolampreys is almost wholly negative, resting inthe total absence of jaws, limbs, and limb-gir-dles. What the mouth was like can only beguessed, but no trace of jaws has yet been foundin connection with it. The most remarkabledistinctive character is found in the presence ofa hard shell, made of bony plates covering theanterior part of the body, while the backboneis developed as a persistent notochord, imper-. BuHhead Shark of California (Gyropleurodus francisd). The recognized families of rays are thePristididce or saw-fishes, the Rhinobatidce orguitar-fishes, the Narcobatidoe or torpedos, theRajidar or skates, the Dasyatidcr or sting-rays,the Myliobatida- or eagle-rays, the Mobulidce ordevil-fishes, and the Ptychodontidtr (extinct)of the Cretaceous. The earliest of these groups,the Rhinobatidce, date from the Jurassic. In the sub-class of Holocephali or Chinicc-roids the upper jaw or pterygoquadrate arcade isimmovably joined to the skull. The teeth arecoalesced into broad plates, and a fold of skincovers the gill-clefts so that there is but oneexternal opening. The vertebral axis is imper-fectly segmented, and the notochord is sur-rounded by partially calcified rings. In all recentgenera, and in most others, there is a strongspine in th
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