RELATIONSHIP OF AMMOCGTES TO OSTRACODERMS 343 
THE RELATIONSHIP OF THE OSTRACODERMS, 
Of the three groups of fishes—the Heterostraci, the Osteostraci, 
and the Antiarcha—the last is Devonian, and therefore the latest 
in time of the three, while the earliest is the first group, as both 
Pteraspis and Cyathaspis have been found in lower levels of the 
Silurian age than any of the Osteostraci, and, indeed, Cyathaspis 
has been discovered in Sweden in the lower Silurian. This, the 
earliest of all groups of fishes, is confined to two forms only— 
Pteraspis and Cyathaspis,—for Scaphaspis is now recognized to be 
the ventral shield of Pteraspis. 
Hitherto a strong tendency has existed in the minds both of the 
comparative anatomist and the paleontologist to look on the elasmo- 
branchs as the earliest fishes, and to force, therefore, these strange 
forms of fish into the elasmobranch ranks. For this purpose the 
sane device is often used as has been utilized in order to account 
for the existence of the Cyclostomata, viz. that of degeneration. The 
evidence I have put forward is very strongly in favour of a con- 
nection between the cyclostomes and the cephalaspids, and agrees 
therefore with all the rest of the evidence that the jawless fishes 
are more ancient than those which bore jaws—the Gnathostomata. 
This is no new view. It was urged by Cope, who classified the 
Heterostraci, Osteostraci, and Antiarcha under one big group—the 
Agnatha—from which subsequently the Gnathostomata arose. Cope’s 
arguments have not prevailed up to the present time, as is seen in 
the writings of Traquair, one of the chief authorities on the subject 
in Great Britain. He is still an advocate of the elasmobranch origin 
of all these earliest fishes, and claims that the latest discoveries of 
the Silurian deposits (Thelodus Pagei) and other members of the 
Ceelolepide confirm this view of the question. 
This view may be summed up somewhat as follows :— 
Cartilaginous jaws would not fossilize, and the Ostracoderms may 
have possessed them. 
They may have degenerated from elasmobranchs just as the 
cyclostomes are supposed to have degenerated. 
Seeing that bone succeeds cartilage, the presence of bony shields 
in Cephalaspis, etc., indicates that their precursors were cartilaginous, 
presumably elasmobranch fishes, 
Of these arguments the strongest is based on the supposed bony 
