A FOSSIL LAMPREY 65 



logical ages. It can, however, show that Cyclostomes are 

 not the degenerate descendants of shark-like forms ; and 

 if only by analogies in the evolution of fishes it may 

 still be able to demonstrate with fair probability their 

 genetic kinships. It may, for example, 

 prove that in the most ancient time there 

 existed undoubted Cyclostomes, and that 

 these in many and most specialized forms 

 were even then branching-off twigs of a 

 great descent tree. In such an event an 

 inference would certainly be the more 

 reasonable which derived the advancing 

 line of fish descent from the genealogical 

 tree of the more primitive Cyclostomes, 

 than that vice versa. 



It is now accordingly of especial inter- 

 est that the fossil remains of what seems 

 undoubtedly a lamprey (Fig. 73) have been 

 discovered in the Devonian ; and this, to- 

 gether with a better knowledge of the 

 ancient and curious chordate group, Os- 

 tracoderms, may, it is hoped, lead to some 

 solution of the Cyclostome puzzle. 



The Ostracoderms 



Fig. 73. The De- 

 vonian Cyclostome, 

 Palaospondylus gunni, 

 T. X 4. (After TRA- 



Ostracoderms, as they are called from QUAIR.) Achanarras 



quarry, N. Scotland. 



their shell-like, dorsal and ventral derm 

 plates, are certainly the oldest known remains of verte- 

 brates.* In their simpler forms they occur in the Upper 

 Silurian ; they flower out in a variety of types in the De- 

 vonian, and shortly become extinct. In the present con- 



* The earlier (Ordovician) vertebrate remains described by Walcott are as 



yet uninterpretable. 

 F 



