Distribution of the Rhabdophora. 279 



Bay, near St. Davids, where the few forms that have 

 hitherto been collected are chiefly Diplograpti and Clado- 

 pliora. I suspect, however, that the oldest strata of Point 

 Levis, with highly complex Dichograptidge, will be found to be 

 on or near this horizon, as well as some of the lowest Skiddaw 

 beds. They may eventually have to be placed at the summit 

 of the Cambrian. 



Middle Arenig. — To this provisional horizon the more 

 typical Arenig beds of Skiddaw and the Lower Graptolite 

 schists of Sweden undoubtedly belong. They appear to be 

 marked generally by the prevalence of the genera Tetragraptus 

 and Didy?nograptus, in combination with an admixture of 

 regular and irregular complex genera of Dichograptidse. 

 The limits of this subformation are as yet undefined either above 

 or below ; but if we regard the Skiddaw and Scanian beds as 

 provisionally typical, we find in this division few Diprionida, 

 and those which are present rise upwards into the succeeding 

 subformation. 



Upper Arenig. — Everywhere in Britain these beds are com- 

 posed of shaly strata varying in tint from light green to 

 black. The Upper Skiddaw of the Lake District possibly 

 includes some horizons not embraced in the more typical and 

 Upper Arenig beds of South Wales, or in the corresponding 

 Scanian strata that overlie the Swedish Or^ocems-Limestone ; 

 but in all these beds the salient character of the Graptolite 

 fauna is the predominance of individuals of Didymogrcvptus, 

 among which the " geminiform " species D. bijidus, Hall, is 

 especially common. No regularly compound genera of the 

 Dichograptidge have as yet been procured from this horizon ; 

 but the irregularly branched forms of the type of Trichograptus 

 fragilis, Nich., are occasionally met with. Diprionida of the 

 families noted above are present, and apparently in greater 

 numbers than in the underlying zone. Phyllograptidee 

 are locally abundant, appearing on this horizon for the last 

 time. 



Our knowledge of the range of the Graptolites within the 

 Arenig formation is as yet too defective to allow us to fix 

 even the approximate range of the species of Dichograptidas. 

 Of the less-understood forms of Diprionida we know hardly 

 any thing with certainty ; and the few recognized zoological 

 facts are deprived of the geological value they would other- 

 wise possess in our catalogue by the undoubted intermixture 

 of species derived from several distinct stratigraphical zones. 



