412 THE SUCCESSION OF THE VERTEBRATE FAUNAS. 



Brunswick, and Newfoundland. Traces of it in the southern 

 hemisphere are also recorded from Queensland. It included 

 unknown fishes which resembled the modern sharks in the 

 structure and general characters of their dermal armour ; for 

 numerous typical shagreen granules (Gcelolepis) and elongated 

 spines some clearly fin-spines (Onchus) are met with in 

 several localities. The fauna, however, chiefly consists of the 

 remarkable group of Ostracoderms, which seem to have been 

 below the rank of true fishes and finally became extinct at the 

 close of the Devonian period, at least in the European and 

 North American areas. The least specialized genus of Pteras- 

 pidse, Cyathaspis, from the Wenlock Limestone of the Island 

 of Gothland and from higher Silurian horizons both in Europe 

 and North America, is the oldest. The Tremataspidse occur 

 in the Island of Oesel in the Baltic Sea ; while the Cephalas- 

 pidse are just beginning to appear (Cephalaspis, Auchenaspis) 

 both there and on the Herefordshire and Shropshire border of 

 Wales. 



DEVONIAN AND OLD RED SANDSTONE. 



There are at least three, if not four distinct and apparently 

 successive fish-faunas in the strata grouped by geologists as 

 Devonian. The earliest differs very little from that of the 

 Upper Silurian, except that the Cephalaspidian Ostracoderms 

 become relatively more numerous, while the Elasmobranch 

 fishes (chiefly Acanthodians) are of more importance and 

 develop into very varied forms. It also contains one new 

 element (Phlyctcenaspis), a forerunner of the highly specialized 

 armoured fishes termed Arthrodira, which are so characteristic 

 of the later Devonian formations. The highest genus of the 

 Pteraspidse, Pteraspis itself, is now abundant ; and when it 

 disappears the family becomes extinct. This fauna is charac- 

 teristic of the Passage Beds and the Lower Devonian Corn- 

 stones of Herefordshire and the adjoining counties, where 

 Pteraspis, Cephalaspis, and Phlyctcenaspis are the commonest 

 genera. It is represented again in the Lower Old Red Sand- 

 stone of the south of Scotland, especially in Forfarshire and 

 Ayrshire ; and the same fauna in the quarries of Turin Hill, 



