164 DE. G. L. ELLES 0:S THE BALA COTJNTEY : [vol. IxXYlii, 



These two forms occur together near the top o£ the Allt-Ddu 

 Mudstones (south-west of Pandj-isaf), so that it appears extremely 

 probable that the Glvn-Gower Beds, together with the Allt-Ddu 

 Mudstones, may be taken to represent the shallow- water equivalents 

 of the zone of Z^icrano^rajytus clingani. 



The general aspect of the Ashgillian fauna, as developed in the 

 Bala district, is widely different from that of the Caradoeian ; 

 though, unfortunately, only the Ehiwlas Beds at the base of that 

 series and the Foel-y-Ddinas Beds at the top contain fossils. The 

 most noticeable feature in the Rhiwlas Beds is the almost complete 

 disappearance of all the big brachiopods which are so numerous, 

 as regards both individuals and sj^ecies, at the lower horizon, those 

 that remain being of small size only. The trilobites and Cystids 

 form the greater part of the fauna, together with numerous small 

 ostracods, but even a casual scrutiny will reveal the fact that these 

 trilobites are of quite different t^^pe for the greater part from those 

 forms in the Caradoeian, although when the highest beds of the 

 Caradoeian are carefully searched a few of these forms may be 

 found, showing that the new fauna came in gradually. Thus, 

 species of Cheionirus, LicJias, Cijhele, JEncrimirus, JRemopleurides, 

 and Siaurocejplicdus are all highh" characteristic of the lowest 

 Ashgillian fauna ; and, of these, specimens of Liclias Jaxatiis, 

 Cyhele rugosa, and Encrimunis sexcosiatiis are not infrequently 

 found at the top of the Caradoeian. Should conditions in an}" 

 area have changed more gradually, a greater admixture of the two 

 faunas might be looked for. 



Prof. J. E. Marr^ has divided the Ashoillian into two. the 

 Phillipsinella Beds below, and the Pliacops-miicronafus Beds 

 above, and in a limited sense his classification may be folloAved in 

 the Bala area. The Ehiwlas Mudstone and Limestone belong 

 unquestionably to the FhiUipsinelJa Beds. This PhiUi-psineUa- 

 farahola fauna is a very varied and highly characteristic one ; a 

 list of the chief fossils found belonging to it is given on p. 172. 



The Foel-y-Ddinas Beds which appear to belong to the Phacops- 

 imicronaius Beds, on the other hand, have a most meagre fauna, 

 and it is probable that only a small ])ortion of the beds containing 

 that fauna are here represented (see list, p. 15G). 



There seems to be a complete passage up from these Foel-y- 

 Ddinas Beds, through the Hirnant Beds, to deposits of undoubtedly 

 Silurian age. The Hirnant Beds contain — locally, at any rate — 

 a distinct assemblage of brachiopods that may be regarded as con- 

 stituting the Orfliis-liirnanfensis fauna (see list, p. 157). This- 

 fauna seems to have been included by Prof. Marr in his Ashgillian, 

 as it occurs in the Ashgill Shales. 



Significant, also, is the occurrence of Glypfograpius ferscidpins 

 in the higher beds of the Bhiwlas Mudstones (Garth Goch), this- 

 graptolite being usually characteristic of the zones of DiceUo- 

 grapiiis anceps or CepliaJograjytiis acianinatus. 



1 Q. J. G. S. vol. Ixxi (1915-16) pp. 192 ei seqq. 



