126 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 21, 



Synopsis of the Silurian fossils as given in Sir R. Murchisorts Work, 



Of 267 species of Mollusca and Articulata described from the Silurian rocks, 

 106 are found in the Ludlow series, and 83 are peculiar to it. 



,, Wenlock limestone, ,, 30 „ 



55 



41 





Wenlock shale, 



„ 21 



75 



>> 



Caradoc, 



„ 40 



39 



>> 



Llandeilo flags, 



„ 15 



Ludlow rocks ...... 



Species common to the various parts of the series in the 



Ludlow. 



Wenlock 

 limestone. 



Wenlock 

 shale. 



Caradoc. 



Llandeilo 

 flags. 



19 

 15 



19 

 ... 

 15 



15 

 15 



io 



5 



2 



7 

 10 



20 



1 



3 

 5 . 



20 



Wenlock limestone 



Wenlock shale 





2 1 7 



1 ! ?> 



Llandeilo flags 













Note. — In this synopsis the corals are rejected as too widely spread ; the Cri- 

 noidea as being imperfect, and therefore difficult of identification, except in the 

 W^enlock limestone ; so that Trilobites and shells only are taken for the comparison. 



It appears from the other tables, that out of fifty-five species in 

 the Wenlock limestone, nineteen are common to the Ludlow series 

 and fifteen to the Wenlock shale ; and that out of forty-one species 

 in the Wenlock shale, fifteen are common to Lower Silurians, ten 

 being Caradoc and five Llandeilo species. 



6. If we take the Wenlock limestone and Wenlock shale together, 

 we have only sixty-four species, nineteen in common wdth Ludlow 

 rocks and nineteen with the Lower Silurians ; or in other words, 

 about one-sixth of the whole Ludlow series, and more than one- 

 fourth of the Lower Silurians. The Wenlock series is therefore 

 truly intermediate; but it is very imperfectly represented in the 

 north of England for want of any rich bands of limestone of the 

 Wenlock age. 



To the comparisons I have instituted in former comnmnications 

 between the Upper Silurian series of Westmoreland and North Wales 

 I have little to add. The lower part of the Denbigh flags appears 

 to be exactly on the same level with the Coniston flags (No. 2), and 

 the whole development of the Upper Silurian rocks in North Wales 

 (with many points of local difference) has many points of general 

 resemblance. The sequence in Westmoreland is however more per- 

 fect than in North Wales, and in neither county is the development 

 at all res^enibling that of Siluria. Beautiful as the sequence of that 

 county is, it is not the true mineral type either for England, Wales, 

 or Ireland. As a general rule, I believe that all limestone bands 

 below the carboniferous series are mere local phaenomena, appearing 

 at intervals, which are perfectly irregular in countries remote from 

 one another. This remark is meant to include Devonian limestones, 

 and all Silurian limestones, both upper and lower, and many other 

 limestones far below those which mark the beautiful sections of Si- 

 luria. Hence we can only identify large subdivisions; and any at- 

 tempt at the' comparison of subordinate parts must often end in 

 positive error. Whether this remark applies to the older rocks of 



