Page:The Scientific Monthly vol. 3.djvu/610

 THE SCIENTIFIC MONTHLY

��auceators of the heavily armored fishes known as the Arthrodira, a group of uDcertaiu relationehips. They have many characters in com- mon with Bothriolepie (joint-neck, dermal jaws, carapace and plas- tron, paired appendages {Acanth- aspis)). Dean, Hussakof and others regard the balance in favor of relationship with the stem of \ the Antiarchi (Bothrwlepis). In

I the Middle Devonian (the Cleve- land shales of Ohio) they attain the formidable size shown in the j Bpecies Dinickthys intermedins.

Like the Ostracoderms these ani- ^ mals are not central or in the main

' lines of fish evolution but repre-

sent collateral lines which early at- tained a very high degree of spe- cialization followed by extinction. The central line of fish evolu- tion is found in the typical carti- laginous skeleton and jaws and four fins of the primordial sharks, the primitive fusiform stage of which appears in the apine-fimied type (Acanthodian) of Upper Si- lurian time. The relatively large- headed, bottom-living types of Rharks do not appear until the De- vonian, during which epoch the early swift-moving, fusiform pre- daceous types branch off into the elongated eel-shaped forms of the Carboniferous. The prototype of the shark group is the Cladose- lache, a fish famed in the annals of comparative anatomy since it demonstrates that the fins of fishes arise from lateral skin folds of the liody with internal stiffening car- tilaginous rods (Fig. 13). which in course of evolution are concentrated to form the central axis of a freely jointed fin, while in a further step of evolution they transform into the cartilages and bones of the limb girdles and limb segments of the four-footed land vertebrate- (Tetra-

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