222 PALEONTOLOGY 



tooth " or placoid scale, the structure and development 

 of which are essentially those of teeth (Fig. 64). Our 

 teeth are in fact the greatly-modified survivors of what 

 was once a general body-covering, retained on the jaws 

 for special purposes when no longer needed on the skin. 



Water-breathing Vertebrata. 



The first British Vertebrata are found near the top of 

 the Silurian system, in the Ludlow " bone-bed." Here 

 abound the little skin-teeth of Thelodus (Fig. 63, a), 

 which, as shown by a complete body (only a few inches 

 long) found in Scotland, is a primitive member of a 

 group, Ostracodenni (Fig. 62), the exact taxonomic position 

 of which is doubtful, as it is not clear whether they 

 possessed the biting jaws found in all other Vertebrata. 

 In higher members of the same class, found also in the 

 Silurian, but more abundantly irnhe Devonian (e.g.,Cepha- 

 laspis) the skin-teeth became fused into large armour- 

 plates, and (by adaptation to a similar life) many of them 

 came to mimic closely the contemporary eurypterids. The 

 microscopic structure of the armour is quite different in 

 the two cases, and though much ingenuity has been 

 wasted in trying to prove an arthropod ancestry for 

 Vertebrata, we may feel confident that the resemblance 

 is only a very striking case of convergence. The ostra- 

 coderms died out at the end of the Devonian. 



The undoubted fishes, with typical vertebrate jaws, 

 have an internal skeleton ; but in the lower of the two 

 grades into which fishes may be divided (Chondrichthyes), 



