20 



THE GEOLO&IST. 



Table I., to whicli attention has so frequently been directed, repre- 

 sents, 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 ascer- 

 tain 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 distributiyity. 



The relative distribution is exhibited in Table lY., which has been 

 calculated from the data contained in Table I., thus : the total num- 

 ber of species in each class is put = 1000, and the figures in the 

 other columns equated to this. 



Sauged in descending order, according to their relative specific 

 prevalence in each er^, the classes stand thus — 



Lower South Devon : Zoophyta, Amorphozoa, Crustacea, Gastero- 

 pQda, Brachiopoda, Bryozoa, Cephalopoda, Echinodermata, and La- 

 mellibranchiata. 



Lower jSTorth Devon : Bryozoa, Brachiopoda, Zoophyta, and La- 

 mellibranchiata. 



Lower Cornwall : Amorphozoa, Crustacea, Zoophyta, Echinoder- 

 mata, Brachiopoda, and Gasteropoda. 



Upper Xorth Devon : Lamellibranchiata, Echinodermata, Bryozoa, 

 Brachiopoda, Gasteropoda, Cephaloda, Crustacea, and Zoophyta. 



Tapper 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. t 



South Devon is the only area in which each of the nine classes oc- 

 curs ; Lower Cornwall and Lower North Devon are each poor in 

 classes as well as species, the latter yielding representatives of four 

 classes only. 



AVhen 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, tliey take the following order : — Zoophyta, Lamellibranchiata, 

 Cephalopoda, and Brachiopoda. 



The class Amorphozoa is the only one in the Devonian Eauua 

 which does not contain either Silurian or Carboniferous species.^ 



From Table Y. it appears that fifty-six genera are peculiar to one or 

 other of the three areas Lower South Devon, L'pper 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. 



* See in Table IV. tlie columns headed " Totals/' 



X See iu Tables I. and IV. the columus headed " Totals." 



+ See iu Table IV. the columns headed "Silurian"' and "Carboniferous." 



