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 m.urchisoni 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 arrangement of armour sud- 

 denly appears in Pterichthys (Fig. 8, page 75) in the middle of 

 the Devonian period, and it persists without essential change 

 until the extinction of Bothriolepis just before the dawn of Car- 

 boniferous times." 



