IV. 20 



HETEROSTRACI 



129 



and lower Devonian. There were ventral as well as dorsal shields (Fig. 

 88), and a long series of gill pouches, but only a single pair of exhalent 

 branchial apertures, suggesting to Watson respiration by a moving 

 flap (velum). The shields were of cell-less bone (isopedin) covered 

 with dentine. The body was covered with scales of similar material. 

 The tail was hypocercal and there were lateral horizontal keels but 

 no fins. Theie were paired eyes, two semicircular canals and clearly 



Fig. 88. Three views of a restoration of Pteraspis. 

 d.sp. dorsal spine; e. eye; m. mouth; r. rostrum. (From White.) 



marked lateral line canals. There was a pineal opening, closed in the 

 adult, but no sign of the nostril, which may have opened into the 

 mouth. The latter was surrounded by long plates, suggesting that it 

 formed a protrusible apparatus, which could be pushed out to form 

 'a kind of scoop or shovel (Fig. 88) whereby mud and decaying refuse 

 could be taken off the bottom, for it seems likely that such were their 

 food and habit' (White). 



The coelolepids or thelodonts are the least known group of agna- 

 thans. The outer surface was covered with fine, placoid-like scales or 

 hollow spines, which in isolation are often found in late Silurian and 

 Early Devonian rocks. The anterior end was usually flattened and 

 wide but the body behind was narrow, with a forked, probably 

 hypocercal tail. Structures that are probably eye-spots occurred 

 widely separated near the front margin. The mouth was ventral and 

 traces of seven branchial arches have been found. There were flap-like 



