290 OUTLINES OF EVOLUTIONAEY BIOLOGY 



with the cyclostomes in the absence of true jaws and (in most 

 known forms) of paired fins, and in the extremely primitive 

 character of the vertebral column. They had also, like the 

 cyclostomes, median fins, but they differed from the latter con- 

 spicuously in the presence of a strongly developed dermal armour, 



FIG. 135. Restoration of Cephalaspis murchiwni, from the Lower Old Red 

 Sandstone, X |. (From British Museum Guide, after Smith Wood- 

 ward.) 



to the existence of which their fossil remains owe their preser- 

 vation. As representative genera of this group we may men- 

 tion Cyathaspis (Upper Silurian), Pteraspis (Lower Devonian), 



FIG. l36.Pterichthys milleri, from the Lower Old Red Sandstone; restored 

 by R. H. Traquair. A. Dorsal; B. Ventral; C. Lateral views, x ^. 

 (From Smith Woodward's " Vertebrate Palaeontology.") 



Cephalaspis (Upper Silurian and Lower Devonian, Fig. 135) and 

 Pterichthys (Lower Devonian, Fig. 136). 



The origin of the Ostracodermi is unknown, but their relation- 

 ship to the cyclostomes has led to their inclusion with the latter in 

 one class, the Agnatha (jawless vertebrates), which stands at the 



