Table I., to which attention has so frequently been directed, represents, so far as is at present known, the absolute distribution of the fossils in the two counties in which they occur; but, for purposes of geological chronology, it is probably of greater importance to ascertain their relative distribution, which may differ widely from that shown by the figures, since the various classes of animals represented in the fossil series were not equally rich in species, and perhaps differed much in, what may be called, their distributivity.
The relative distribution is exhibited in Table IV., which has been calculated from the data contained in Table I., thus: the total number of species in each class is put = 1000, and the figures in the other columns equated to this.
Ranged in descending order, according to their relative specific prevalence in each era, the classes stand thus:[1]—
Lower South Devon: Zoophyta, Amorphozoa, Crustacea, Gasteropoda, Brachiopoda, Bryozoa, Cephalopoda, Echinodermata, and Lamellibranchiata.
Lower North Devon: Bryozoa, Brachiopoda, Zoophyta, and Lamellibranchiata.
Lower Cornwall: Amorphozoa, Crustacea, Zoophyta, Echinodermata, Brachiopoda, and Gasteropoda.
Upper North Devon: Lamellibranchiata, Echinodermata, Bryozoa, Brachiopoda, Gasteropoda, Cephaloda, Crustacea, and Zoophyta.
Upper Cornwall: Cephalopoda, Lamellibranchiata, Gasteropoda, Brachiopoda, Bryozoa, Crustacea, Echinodermata, and Zoophyta.
Both relatively and absolutely each class has its maximum specific development in South Devon, with the exception of Lamellibranchiata only, which has its greatest specific variety in Upper North Devon.[2]
South Devon is the only area in which each of the nine classes occurs; Lower Cornwall and Lower North Devon are each poor in classes as well as species, the latter yielding representatives of four classes only.
When ranged in descending order, so as to show, relatively, the transmission of species from the Devonian to the Carboniferous era, the classes stand thus:—Bryozoa, Echinodermata, Brachiopoda, Gasteropoda, Cephalopoda, Crustacea, and Lamellibranchiata. And when similarly arranged for the species derived from the Silurian Fauna, they take the following order:—Zoophyta, Lamellibranchiata, Cephalopoda, and Brachiopoda.
The class Amorphozoa is the only one in the Devonian Eauna which does not contain either Silurian or Carboniferous species.[3]
From Table IV. it appears that fifty-six genera are peculiar to one or other of the three areas Lower South Devon, Upper North Devon, and Upper Cornwall; and that, of these, forty-six, or very nearly one-half the total ninety-seven, are restricted to Lower South Devon. No genus is confined to Lower North Devon or Lower Cornwall.
