. Devonian fishes of Iowa. Fishes, Fossil; Paleontology; Paleontology. 58 IOWA GEOLOGICAL SURVEY and somewhat later rocks in Lanarkshire. There is, indeed, no doubt that the granular armour was the "fashionable" fish- skeleton of Upper Silurian time. It soon became usual, however, for the skin-tubercles to fuse together into groups, and in the earliest Devonian faunas the most common Ostracoderms are those like Cepkalaspis and Pteraspis. The first of these (shown in Fig. 4) is especially in- structive as showing how the tubercles became plates, and how the shape of these plates depen
. Devonian fishes of Iowa. Fishes, Fossil; Paleontology; Paleontology. 58 IOWA GEOLOGICAL SURVEY and somewhat later rocks in Lanarkshire. There is, indeed, no doubt that the granular armour was the "fashionable" fish- skeleton of Upper Silurian time. It soon became usual, however, for the skin-tubercles to fuse together into groups, and in the earliest Devonian faunas the most common Ostracoderms are those like Cepkalaspis and Pteraspis. The first of these (shown in Fig. 4) is especially in- structive as showing how the tubercles became plates, and how the shape of these plates depended on the nature of the under- lying parts of the body. In the head-armour of Cephalaspis a few regularly spaced tubercles grew larger than the others, and each of these became a center of attraction with which the im- mediately surrounding tubercles coalesced, by the thickening of their base, to form polygonal plates. Where the underlying soft parts were not in constant motion these polygonal plates fused again into a continuous shield; while in the roof of parts, such as the presumed gill-chambers, where flexibility was needed, the plates remained as a loose mosaic, which is often lost in the fossils. .... Fig. 4. '•TSFig. 4. Cephalaspis Egert. Lower Old Red Sandstone; Herefordshire. Headshleld seen from above, tail twisted to show dorsal fln and heterocercal tail mainly in side-view, x I (after Smith Woodward). The latest "fashion" among the Ostracoderms of the Devonian period consisted in an armour of symmetrically arranged over- lapping plates on the top of the head and round the body, with a pair of flippers similarly armoured and appended to the latter. Here the the primitive skin-tubercles seem to have fused, not into polygonal plates, but along the lines of the slime-canals which traverse the skin of many of the Ostracoderms, though unfortu- nately none of the early stages in the process have hitherto been discovered. So far as known, this arra
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