112 PROCEEDINGS OE THE GEOLOGICAL SOCIETY. 



Hydrozoa first appeared in the British Arenig sea, and that with- 

 out any known previously-existing Ccelenterate fauna. We have no 

 clue to the area or region whence they migrated ; possibly Canada 

 may be selected as one, at least, of the probable areas for their 

 origin and dispersion. We regard the Quebec group of rocks as the 

 equivalent in time of our Arenig series ; and they contain the same 

 species of Graptolites in great profusion. We cannot look to Scan- 

 dinavia for such an assemblage; and although the Bohemian Grap- 

 tolitic fauna was extensive, still, on the whole, I should regard the 

 Canadian, through certain genera and species, as being the source 

 of dispersion and migration. There is intimate agreement between 

 the faunas of many of the American and our own Palaeozoic rocks ; 

 and this is notably the case with the Arenig and its Graptolites. 



Thirteen genera and 28 species of Graptolites occur in the 

 Skiddaw group, showing that more than two thirds of the known 

 genera and species are in the almost barren slates of the Keswick 

 area ; for, with the exception of 10 species of Annelida (all tracks 

 and burrows), 9 species of Crustacea, and 2 Brachiopoda, no 

 other forms are known — neither Lamellibranch, Gasteropod, Pte- 

 ropod, or Heteropod, so far as we know, ever having appeared 

 in the Skiddaw series west of the Stiperstones in the Shelve area*. 

 The Arenig rocks are there clearly exhibited, but yield only a 

 small series of Hydrozoa. Only 3 genera and 6 species have yet 

 been collected ; they are Diplograptus pristis ?, Didymograptus pa- 

 tulus, D. geminus, D. Jiirundo ?, D. constrictus, and Clematograptus 

 implicatus. The Crustacea eminently characterize the Shelve 

 Arenigs ; and it is through the 6 genera and 9 species of Trilobita 

 and the stratigraphical position that the age of the beds is clearly 

 determined : 3 of the 7 genera that occur here are also in the Skid- 

 daw rocks — Agnostus Morei, Ogygia Sehvynii, and JEglina binodosa. 

 Next to North Wales the Shropshire Arenigs contain the smallest 

 number of species ; but they are distinctively characteristic. 



North Wales, from the Tremadoc and Efestiniog areas at Tre- 

 madoc, Ty-obry and Tai-hirion, has yielded only 4 genera and 4 

 species. 



Next to the Skiddaw rocks and the Westmoreland area the chief 

 development of the Arenig group occurs in South Wales on the St.- 

 David's promontory, at Llanvirn, Whitesand Bay, Trwyn-hwrd- 

 dyn, and Porth Melgan, and the rich locality of Ramsey Island. 

 These several localities have yielded, to the researches of Dr. Hicks, 

 Mr. Hopkinson, Mr. D. Homfray, and others, about 8 genera and 

 18 species of Graptolites. Only 3 species are common to North and 

 South Wales, namely Diplograptus dentatus ?, Climacograptus con- 

 fertus, and Glossograptus ciliatus ; 6 show the relation of the Skiddaw 

 rocks with South Wales, viz. Didymograptus affinis, D. bifidus, D. 

 patulus, Tetragraptus quadribrachiatus, Pliyllograptus typus, and 

 Diplograptus dentatus ? ; and 7 are common to the Stiperstones area 

 and South Wales, viz. Didymograptus patulus, D. affinis, D. hirundo, 

 D. bijtdus, Diplograptus dentatus, Clematograptus implicatus, and 

 * Lords Hill, Bog Mine, Dingle, Ritton Castle, Corndon, &c. 



