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. 
Fig. '73. — The De- 
vonian Cyclostome, 
The Ostracoderms Paleospondylus gunni, 
T. xX 4. (After TRA- 
Ostracoderms, as they are called from Quai.) Achanarras 
their shell-like, dorsal and ventral derm 7” ee 
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 
