166 PROF. o. T. JoxEs OX [vol. Ixxvii, 



fossils which are elsewhere distinctive of Lower or Upper Yaleiitian 

 are recorded from the same strata. The faunal relations are, 

 therefore, too obscure at present to allow of a correlation with 

 other districts being made. 



YIII. Other Areas. 



The remaining areas of Valentian rocks have this in common, 

 that no recognizable equivalent of the Lower Llandovery Group 

 has been proved. Where the base of the series is exposed, it rests 

 on various older rocks rano'in^ from the Pre-Cambrian to the 

 Caradocian. In other areas the base is not exposed: but the 

 evidence of surrounding districts leads to the inference that the 

 Silurian rests unconformably on the lower rocks. 



The development is subject to marked variation in thickness and 

 faunal chai-acters ^vhich has commonly been accounted for by 

 regarding it as a littoral facies of the series. This appears to be 

 £i sufficient explanation. On faunal grounds we inay recognize 

 two sub-facies which have different geographical distribvitions : 

 (1) The Penta merits ohlongus sub-facies, marked by the great 

 abundance of that species, and characteristic of the Caradoc, 

 Presteign, Bailth, AVoolhope. Malvern, May-Hill, Lickey, and 

 Posemarket districts. (2) The Sfroplieoclonta compressa sub- 

 facies, which includes the remaining areas of the South- West of 

 England and South Pembrokeshire ; namely, Tortworth, the Men- 

 dips, and the Marloes-Wooltack district. The boundary between 

 these two facies is sharply defined in Pembrokeshire, where they 

 are not more than 3 to 4 miles apart, and ranges thence almost due 

 east and west, between Tortworth and May Hill. On geographical 

 grounds another classification of the littoral development into a 

 northern and southern sub-facies is thus possible. It is a fact of 

 considerable interest that the contempoi'aneous volcanic eruptions 

 are confined to the area of the southern sub-facies. 



(1) The Pentamer'us ohlongus or Northern Sub-Facies. 



This is most typically represented in the Caradoc region, where, 

 as also at Builth, its relation to the Wenlock Shale can be observed. 

 In each district a thin group of green and purple shales (or black, 

 as at Builth) intervenes between the abundantly fossiliferous sand- 

 stones and the Wenlock Shale. In Pembrokeshire the Eosemarket 

 Stage is overlain uncomformablv bv Old Red Sandstone, and no 

 younger rocks are exposed in that inlier. Throughout most of the 

 remaining districts the Yalentian rocks pass up conformably into a 

 calcareous group called the Woolhope Limestone, which is 

 generally assigned to the Wenlock Series. 



The usual associates of P. ohlongus in these areas are Afrypa 

 reticularis, CoeJospira Jiemisplierica, SclniclierteUa pecten. and 

 Plectamhonites — also usually StricMandinia lens and S. lirata. 

 These are common forms in the Millin Stage of Haverfordwest 

 and the Upper Yalentian of Girvan. Stroplieodonta compressa 

 usually occurs also. The purple shales have yielded a considerable 



