I30 PROCEEDINGS OE THE GEOLOGICAL SOCIETY. 



It has been stated that the fauna of the Llandeilo rocks num- 

 bers 80 genera and 175 known species. The Caradoc or Bala 

 contains 179 genera and 613 species; or 88 new genera and 375 

 new species make their appearance either at the close of the 

 Llandeilo or during that period which ushered in or brought about 

 those changes of conditions under which the deposits of the Cara- 

 doc sea were accumulated and its species developed and multiplied, 

 covering as it does so extensive an area in Europe and Britain, also 

 extending to America, whether homotaxially in that region or con- 

 temporaneously with the deposition in the British area it is difficult 

 to say. That the Trenton and Hudson-River group of North 

 America, and nearly the whole of the Etage D of Barrande (Bohemia) 

 and Regio D of Angelin (Sweden), are the equivalents of our great 

 Middle Caradoc and Bala, or the true Caradoc, is now not doubted. 

 The fauna of the Bala group, especially in its zoological groupings, 

 differs much from the preceding Llandeilo, although many species 

 (85) are common to the two formations. 



I purpose treating of the Caradoc rocks and their fossils at some 

 length — their stratigraphical place, the geographical distribution of 

 their fossils (especially with reference to the succeeding Lower and 

 Upper Llandovery) — questions which demand critical examination 

 or analysis. 



The three classes having the greatest number in common are the 

 Hydrozoa (Rhabdophora), the Crustacea (Trilobita), and the Bra- 

 chiopoda. The number of known species in the Llandeilo in each 

 of the above classes is 



Hydrozoa 44 



Crustacea 45 



Brachiopoda 34 



In the Caradoc fauna the same three classes number, including 

 those from the Llandeilo, 



Hydrozoa 38 



Crustacea 146 



Brachiopoda 109 



Numerically, the remaining ten classes have little value for compa- 

 rison, although the Actinozoa, Echinodermata, Lamellibranchiata, 

 and Gasteropoda are well illustrated (see Table X.). It is through 

 the above three extensively developed groups, which are mainly 

 moderately deep-sea forms, that the two formations are united. 



The fact of 73 species out of 175 passing from the Llandeilo to 

 the Caradoc is highly suggestive, and is confirmatory of the views of 

 Sedgwick in uniting the Llandeilo to the Lower Bala; and the 

 distinctiveness of the great Middle Bala is confirmed by the fact 

 that, out of 610 species in that formation, only 102 pass to the Lower 

 Llandovery. Therefore, on palseontological data, we may regard the 

 Llandeilo and Caradoc as being most closely allied. The 102 trans- 

 gressive Caradoc species are distributed through the following 

 classes : — 



