RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 343 



The Eelationship 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 7 recognized to be 

 the ventral shield of Pteraspis. 



Hitherto a strong tendency has existed in the minds both of the 

 comparative anatomist and the palaeontologist 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 

 same 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 Paget) and other members of the 

 Ccelolepidse 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 



