GIRTY, GEOLOGIC AGE OF THE BEDFORD SHALE 299 



species" cannot be taken with the first meaning (a), because some types 

 found in the Carboniferous have a very long range and originated at 

 much lower horizons. Leptcena rhomb oidalis is an example. 8 



If we assume the second meaning (6) for the expression "Carbon- 

 iferous species," the principle would mean frankly a redetermination of 

 the Devonian-Carboniferous boundary, not only in other areas but in the 

 typical area as well, a redetermination, moreover, which would never 

 cease, because the Devonian beds which would thus be added to the Car- 

 boniferous would carry over other species having a sporadic appearance 

 at lower horizons. This would entail a new adjustment and so on appar- 

 ently until the bottom of the stratigraphic column was reached. Fur- 

 thermore, is it one such species, or two, or a score that the application of 

 this principle involves ? Eeasonably but one. The decision might easily 

 then come to depend on the identification of one or two specimens re- 

 sembling several closely related species. 



If we now take the last meaning (c) for the expression "Carboniferous 

 species," i. e., species characteristic of the Carboniferous, those not known 

 to occur at any horizon outside of the Carboniferous, the statement under 

 consideration becomes hardly more than a truism, but a truism which 

 assumes that we have complete knowledge of the range of species, that 

 the species in question are not only not. known to range outside of the 

 Carboniferous but that they cannot so range, and this is an assumption 

 which everyone knows is quite inadmissible, since the range of species as 

 recognized at any time is continually being changed by new data. Other 

 considerations might be brought forward, but it is already plain that this 

 is not a workable principle, no matter how understood, for determining 

 the boundaries between systems, or for classifying the Bedford shale. 



It might be held, on the other hand, that the proper way to fix the 

 line between the Devonian and Carboniferous systems is by the disap- 

 pearance of the last Devonian forms. This principle is the antithesis 

 of that just considered. It is open to the same objections and is equally 

 untenable. 



The only practicable method of interpreting paleontologic data in 

 most cases of this sort is evidently by taking the balance of evidence. By 

 this method the decision hangs not upon one or two forms but upon the 

 entire number known, and, although the evidence of each form may be 

 impaired by poor material and close relationship between species, it be- 

 comes cumulative. Here the expressions "Carboniferous" and "Devo- 

 nian" species signify species especially common in their respective sys- 



8 De Koninck, however, distinguishes the Mississippian Leptsena as a distinct species 

 under the name Leptwna analoga. 



