520 HISTORICAL PALEONTOLOGY. 



which constitute the genera Thelodus and Sphagodus. These 

 spines are believed to indicate the existence in the Upper 

 Silurian seas of Cestraciont fishes allied to the living Port- 

 Jackson Shark, whilst the latter may have belonged to some 

 form like the existing Dog-fishes. It must be admitted, how- 

 ever, that the true nature of these fossils is still open to ques- 

 tion. From the Upper Silurian series of Bohemia M. Bar- 

 rande describes no less than five fishes viz., Coccosteus primus, 

 C. Agassizi, Asterolepis Bohemicus, Gompholepis Panderi, and 

 Ctenacanthus Bohemicus, of which the first four belong to the 

 Ganoids, whilst the last is supposed to be a Cestraciont or a 

 Selachian. 



CHAPTER XLIX. 



DEVONIAN PERIOD. 



ROCKS OF THE PERIOD. 



THE Silurian Rocks are succeeded upward by a great system of 

 rocks, mainly of the nature of sandstones and conglomerates, 

 to which the name of Old Red Sandstone has been applied. 

 The name Devonian formation is also employed to designate 

 these same strata, rocks supposed to belong to this period 

 being largely developed in Devonshire, in England. It is 

 probable, however, that the Devonian rocks represent a por- 

 tion only of the Old Red Sandstone, and that they cannot be 

 regarded as the full equivalent of the Old Red Sandstone of 

 other regions. The term " Devonian " may, however, when 

 thus understood, be usefully employed as a general term for 

 all the strata which intervene between the Silurian System and 

 the succeeding formation of the Carboniferous rocks. 



The uncertainty as to the exact position of the Devonian 

 Rocks of Devonshire in the series of the Old Red Sandstone, 

 or the uncertainty as to whether they represent the Old Red 

 Sandstone in whole or in part, arises from this that though 

 both formations are fossiliferous, the peculiar fossils of each 

 are never found associated together. The peculiar fossils of 

 the Old Red Sandstone proper are not found in the rocks of 

 Devonshire ; and the fossils of the latter, though found in 

 equivalent strata on the Continent of Europe, do not occur in 

 the beds to which the name of Old Red Sandstone was origi- 

 nally applied. This, however, may be largely due to the fact 



