A FOSSIL LAMPREY 



65 



^.V>?^ 



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. y^) 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 Ostracoderuis 



Fig. 73. — The De- 

 vonian Cyclostome, 

 Palceospondylus gunni, 

 T. X 4. (After Tra- 



Ostracoderms, as they are called from quair.) Achanarras 



, . , 1, ■,., , 1 , 11 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. 



