1866.] 253 ^^^"■ 



sedimentary formations, between the Hamilton group and the Coal 

 measures in the east, and between the same group and the Burlington 

 (Carboniferous) limestone in the west, the Devonian aspect of the 

 fauna on the one hand, and its Carboniferous aspect on the other, are 

 due in a great degree .to geographical and physical conditions, and 

 not to difference in age or chronological sequence of the beds contain- 

 ing the fossils. 



This view of the case, which is consonant with the facts observed, 

 will account for the coming in of forms which we term Carboniferous, 

 as we pursue our investigations to the westward. 



The same opinions seem gradually to be obtaining ground in Great 

 Britain, but the idea is not new with me. It is now about fifteen 

 years since I expressed similar opinions in a review and comparison 

 of the Palaeozoic groups and systems of Europe and America.* 



In some of the concluding paragraphs of this chapter, after having 

 made a comparison of the species cited as common to the Silurian 

 and Devonian and to the latter and the Carboniferous system, I have 

 said : 



" The arenaceous and argillaceous deposits, which we trace uninter- 

 ruptedly over so wide an area, and which present to us such gradual 

 and almost imperceptible changes in the fauna when studied con- 

 tinuously, would, if broken up and isolated so that they could not be 

 traced consecutively, present the same phases which are exhibited by 

 the systems in Europe to which they are related. From all these 

 facts there seems to be but one conclusion, and that is, in the British 

 Islands particularly, either there are remarkable exceptions to the 

 general law in the continuation of species from one to another, or that 

 there is no foundation for a distinction between the Devonian and 

 Carboniferous systems." 



Note. — The Spirifera aUa referred to in the preceding pages is 

 an analogue of the European carboniferous Spirifera cuspidata, 

 having a similar elevated area which is usually " slightly inclining 

 forward or nearly rectangular to the general plane of the dorsal valve. 

 The fissure is high and narrow, and is closed for two-thirds of its 

 length from the apex by a concave septum which is entirely indepen- 

 dent of the pseudo-deltidium." On page 249 of vol. iv, Pal. N. Y., 

 I have made the following remarks under the description of the 

 species : 



* In Foster and AVhitney's Report, chap, sxiii, pp. 285-318. 



