HABITAT OF THE EARLY VERTEBRATES 401 



pre-Devonian history, and to reenforce the conviction that the 

 evolution of full suites of armor and varied forms of dentition 

 was the work of a prolonged period and had almost necessarily 

 many fossilizable stages previous to the striking display in the 

 Devonian period and this conviction becomes the" more firm 

 when it is considered that the differentiation and the armoring 

 extended not only to many different orders, but to subclasses 

 and even classes. If Walcott's interpretations be accepted to 

 relieve the dearth of the record, they must also be accepted as 

 showing susceptibility to fossilization so much the earlier. 



As the record now stands, there are fragments of plates, scales, 

 and a supposed notochord from the Trenton of Colorado; but a 

 dearth everywhere else in the widespread and well-studied 

 Ordovician and throughout the early and middle Silurian. Then, 

 in the transitional stages from the Silurian to the Devonian, fish 

 remains appear on both continents, and before the Devonian has 

 passed, they present a rich and varied deployment, embracing 

 not only the two classes Agnatha (jawless fishes, Cyclostomatd) 

 and Pisces (true fishes) but all the known subclasses of these 

 and a majority of their orders, according to the most recent 

 authoritative classification. 



The physical and biological associations of this extraordinary 

 deployment were peculiar. On neither continent do fish remains 

 appear abundantly in the open sea deposits. They are confined 

 chiefly to the sediments of inland waters or of littoral zones or 

 of embayed arms of the sea. The fish of the Corniferous lime- 

 stone, perhaps most nearly an exception to this generalization, 

 may properly be put in the last class, for the Corniferous beds 

 were laid down in a great bay with only limited connection with 

 the sea, though the fauna was truly marine. 



In the Ludlow "bone bed" of England, where they first 

 make their appearance in abundance, the fish remains are associ- 

 ated with Eurypterids, probably the most gigantic Crustaceans 

 that have ever lived, some of them attaining two meters in length. 

 There is the same association on the continent, notably in the 

 island of Oesel in the Russian Baltic and in Podolia and Galicia, 



