1846.] SEDGWICK ON THE SLATE ROCKS OF CUMBERLAND, ETC. 125 



The anomalous position of the masses has been probably caused by 

 great faults and partial dislocations. I here subjoin a comparative 

 view of the fossils in the Upper and Lower Silurian rocks of the 

 country here described. 



Comparative view of the Fossils in the Upper and Lower Silurian 

 Rocks of Westmoreland, 



Q J Species peculiar to Common Species peculiar to 



Upper Silurian. species. Lower Silurian. 



Pisces 3? ... — ... — 



Cephalopoda 17 ... 1 ... 3 



Gasteropoda, Pteropoda,&c. 17 ... 1.'' ... 2 



Lamellibranchiata 34 ... — ... — 



Brachiopoda 15 ... 2 ... 19 



Tunicata 2 ... — ... 1 



Crustacea 5 ... 1 ... 11 



Annelida 4 ... — ... 3 



Echinodermata 4 ... — ... 2 



Zoophyta 7 ... 5 ... 11 



Total peculiar to Upper S. 108 ... 10 ... 52 



— common species 10 



— peculiar to Lower S. 52 



T^+T80'= 350 species j^" ^«f ^^'d '"' 

 To these may be added from other localities 1 1 .« • ^ v> estmoreiana. 



about I lou species. 



Probable total of British Silurian species \ ^ QJ^^ 



described \ 



When it is considered what a fossil impress is given to these old 

 rocks by the Cephalopoda, Gasteropoda, Lamellibranchiata and 

 Brachiopoda, and that of these four families we have eighty-three 

 species in the Upper Silurian rocks, and twenty-four in the lower, 

 and only three species in common, the list shows a most marked dif- 

 ference between the two systems. On the whole, the list indicates 

 a progress towards a higher organic structure as we ascend from 

 the lower to the upper rocks. Again, the Crustaceans peculiar to 

 the old rocks began in the earliest time, reached their maximum of 

 development at a very early period, and then began to decline, so 

 as not to pass the carboniferous epoch. Lastly, I may remark that 

 the difference indicated in the list between the Upper and Lower Si- 

 lurian fossils of the Lake mountains is probably greater than will be 

 found true on further examination ; because most of the fossils have 

 been derived from the two extremes of the general section, viz. 

 No. 1 and No. 5 of the tabular list, the intermediate parts not having 

 been so well explored by the fossil collectors. 



In confirmation of the preceding view, I subjoin another interest- 

 ing table, supplied by Mr. Salter from Sir R. Murchison's descriptive 

 lists. 



* Several new species have been discovered since this list was made out. 



