PENGELLT—rOSSILS OF DEYON AT^D CORNWALL. 



23 



The genera found in the two counties were not all confined to the 

 Devonian period. The following table shows their Chronological dis- 

 tribution so far as the Silurian, Devonian, and Carboniferous deposits 

 of Britain are concerned. 



TABLE VI. 





03 





Common to 





ener 



i 



a 











O 

 a 



'S 



Devoi 





- 9 



3 - 



US an 





o 

 > 



o 



I- 



ifero 

 an. 



ifero 

 nian 





Q 





3 3 



a 0 u 



c 0 





'3 



o 







i 



la 







PM 



P 



6 







4 



3 



1 







Zoophyta 



20 



10 



5 



"3 



2 



Echinodermata 



6 



2 





3 



1 



Crustacea 



8 



1 



5 





2 





7 







"5 



2 



Brachiopoda 



16 



"i 



"2 



8 



3 



Lamellibranchiata . . . 



17 



2 



1 



9 



5 



Gasteropoda 



14 



2 





11 



1 



Cephalopoda 



5 



1 





2 



2 



Totals 



97 



24 



14 



41 



18 



From which it appears that twenty-four genera, — about one-fourth 

 of the whole series, — are peculiar to Devonian deposits, fourteen 

 common and restricted to the Silurian and Devonian, forty-one com- 

 mon to all three, and eighteen common and restricted to the Devonian 

 and Carboniferous ; hence a total of fifty-five Devonian genera occur 

 in the preceding, and fifty-nine in the succeeding period. Some of 

 the genera occur in Neozoic deposits, and a few in the existing 

 Fauna. 



When the numbers of species contained in each of the forty-one 

 genera of tlie fourth column (Table VI.) are tabulated in parallel 

 columns for the three periods, the figures present themselves in four 

 different principal forms of succession, as may be illustrated by taking 

 the genera JFavosites, Cyathophyllmi, Loxonema, and Orthoceras. 



Sil. Dev. 



Eavosites 8 5 



Cyathophyllum 9 14 



Loxoneina 2 8 



Orthoceras 55 12 



Carb. 



1 a descending-descending series. 



8 ascending-descending. 

 14 ascending-ascending. 

 35 descending-ascending. 



The first kind shows that the maximum specific development occurs 

 m the Silurian era, the second in the Devonian, and the third in the 

 Carboniferous ; the fourth kind may perhaps be regarded as a sort 



