I38 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 









m 

























SP 











CD 



t> 









a 



d 



1 









6 





t> 

 O 













Longing 



Menevi; 



Lingula 



Tremad 



Arenig. 



Llandei 







«! 

 PS 



O 



L. Llan 



U. Llan 



Wenloc 



Ludlow 









1 



1 



3 



7 



14 



41 



20 



10 



16 



7 















4 



8 



4 



4 



4 



1 



























Strophomena 













2 



19 



11 



11 



16 



7 



Rhynchonella 













1 



8 



7 



9 



10 



5 









1 



1 



3 



7 



21 



76 



42 



34 



46 



20 



Those genera having only one or two species are doubtless of 

 equally high stratigraphic significance. Such are Orihisina, Sipho- 

 notreta, Poramhonites, Triplesia, Merista, and Meristella. 



Lamellibranchiata.- — The known species of Biralvia in the 

 Caradoc and Bala rocks are more numerous than in any other 

 Lower Palaeozoic group. Thus the Tremadoc rocks of St. David's 

 yield to us the earliest Bivalve fauna, consisting of 5 species ; the 

 Arenig has 6, the Llandeilo 6, the Caradoc 76, the Lower Llandovery 

 only 3, the Upper Llandovery 29, the Wenlock 45, and the Ludlow 

 71. These numbers help us to see, by comparison, the zoological 

 value of the class Conchifera in the several strata. 



Ctenodonta, Orthonota, Modiolopsis, PUrinoea^ Ambonychia, and 

 Palcearca are the largest genera, or those having the most species, 

 by comparison with the older and younger'formations. The Caradoc 

 Conchifera excel in number all beneath the Carboniferous. The 

 following Table shows this, through the 6 Caradoc genera above 

 named : — 





Longraynd. 



Menevian. 



Lingula-flags. 



Tremadoc. 



Arenig. 



Llandeilo. 



Caradoc. 



Lower Llandovery. 



Upper Llandovery. 



Wenlock. 



Ludlow. 



Ctenoclonta 









2 





1 



17 





6 



3 



4 



Orthonota 















5 



1 



6 



3 



16 



Modiolopsis 









4 



"i 



2 



16 





3 



6 



5 



Pterinsea 















6 



1 



6 



11 



12 



Ambonychia 















8 







2 



1 



Palaearca 











"2 



"i 



5 



















6 



3 



4 



57 



2 



21 



25 



38 



Cardiola with 3 species, Mytilus with 4, Pleurorhynchus, Ano- 

 dontojosis, Cucullella, Megalomus, Clidophorus, Area, and Lyroderma 

 having each 1 species, complete the Conchifera of the Caradoc. Their 



