98 



REPORT 1860. 



Each area is marked by some class being relatively most abundant in it, as is ex- 

 hibited below, thus* : — 



When ranged, in descending order, so as to show, relatively, the migration of their 

 species from the Devonian to the Carboniferous era, the classes stand thus : Bryozoa, 

 Echinodermata, Brachiopoda, Gasteropoda, Cephalopoda, Crustacea, Lamellibran- 

 chiataf. 



The distribution of the genera is just as marked as that of the species ; 92, out of 

 the total 97, are found in Lower South Devon, and of these 45 are, in Britain, pecu- 

 liar to it. Every genus of the classes Zoophyta and Brachiopoda occurs in it; the 

 genera thus limited, however, are usually poor ; 33 of the 4.5 contain each but a 

 single Devonian species. The richest genus not found elsewhere is Acervularia, a 

 group of corals belonging to the great Palaeozoic family Cyathophyllidse ; it contains 

 7 species, all peculiar to this area. One of the richest genera in the entire series 

 is Clymenia, belonging to the class Cephalopoda; of this genus 11 species are found 

 at South Petherwin, and not one is met with elsewhere in Britain. The genus Cyrto- 

 ceras, with the single exception of C. rusticum (probably a synonym for Orthoceras 

 arcuatum), is restricted to Lower South Devon, where it is represented by 1 2 species. 



The chronological range of the Devonian genera of Devon and Cornwall is shown 

 below. 



Table V. 



Some of the genera common to the Devonian and Carboniferous eras, are found 

 also in more recent deposits, and even in the existing fauna. 



Such appear to be the prominent facts in connexion with the subjects under con- 

 sideration. What is their interpretation? This is a problem more easily proposed 

 than solved. Are we to believe that our knowledge of the geological record is too 

 imperfect to warrant any important generalizations ? Do our museums fully repre- 

 sent the fossilized remains of bygone forms of life? Have all the extinct organisms 

 already in our possession been registered in the published lists? Is the record itself 

 so incomplete as to be altogether incapable of revealing to us the physical and organic 

 history of our planet? Are the notions of biologists respecting specific distinctions 

 sufficiently mature and uniform to warrant a reliance on their decisions? Some- 

 thing must doubtless be conceded on each of these points ; still there cannot but be a 



* See in Table IV. the columns headed " Totals." 



f See in Table IV. the column headed " Carboniferous." 



