Yol. 62.] TARANNON SEKIES OF TARANNON. 697 



include not only the Brynmair Shales, but also the Gelli Shales, 

 for M. rimcinatus and M. turriculatus occur in association with 

 M. eociguus and M. discus ; indeed, M. discus is said l to be the most 

 characteristic species of the zone. It is probable, therefore, that the 

 highest beds of the Birkhill (namely, the zone of Rastrites maximus) 

 are absent in Scania. 



Intervening between the zones of M. runcinatus and C. Murchisoni 

 there are in Scania, according to Tullberg, four zones, three of which 

 are characterized by species of Oyrtograptus. The number of species 

 hitherto detected in these upper zones in Scania is so small, that it 

 is not easy to institute a very close comparison ; but they can be 

 paralleled, in a general way, with our Tarannon zones. His zones 

 of Cyrtograptus Grayce and C. (Monograptus) spiralis probably 

 correspond to the Dolgau Mudstones, for Monograptus cremdatus is 

 an abundant species in both. The latter zone may be paralleled 

 fairly accurately with the fossiliferous band at the top of the Dolgau 

 Group found in the northern part of the district. If Cyrtograptus 

 La,pworthi is merely a young form of 0. Murchisoni, then, in our 

 opinion, both the zones named after these species should be included 

 in the lower part of the jSTant-ysgollon Shales. Therefore, in Scania, 

 the division between the Rastrites-Skiffer and the Cyrtograptus- 

 Skiffer occurs about the middle of our Tarannon Series. 



It will be observed that, in the Tarannon district, the highest zone 

 of the Llandovery Series is that of Monograptus SedgivicJcii. In his 

 paper on the Moffat Series (1878), Prof. Lapworth distinguished a 

 still higher zone in his Birkhill Group, namely, that of Rastrites 

 maximus. In his later paper, however, ' On the Geological Distri- 

 bution of the Rhabdophora ' (1880), while he groups the Birkhill 

 Shales as a whole with the Llandovery, he points out (p. 200) that 



' the zone of Rastrites maximus must be regarded as the zone of transition 

 into the succeeding formation. Its fauna is essentially a compound of those 

 characteristic of the more strikingly-separated beds above and below. It ought, 

 in all probability, to be regarded as forming the base-line of the Tarannon 

 Group ' ; 



and in the Table at the end of that paper (p. 201), he groups the 

 zone of Rastrites maximus as the lowest zone of the Tarannon 

 formation. 



It has, nevertheless, been the frequent habit of stratigraphists to 

 include the whole of the R.-maximus zone with the Llandovery. 

 But, so far as the evidence — strati graphical and pakeontological — 

 goes in the Tarannon district, the beds with R. maximus certainly 

 form the base of the Tarannon Series, and the division-line 

 between our Llandovery and Tarannon must be drawn at the top 

 of the zone of Monograptus SedgwicJcii. This also appears to be 

 the most convenient stratigraphical line at Ehayader and at Conway, 

 and it will probably be found to be generally the case elsewhere. 



1 S. L. Tbrnqu^t, ' On the Diplograptidae & Heteroprionidse of the Scanian 

 I'asfrites-Beds ' Kongl. Fysiogr. Sallskapets i Lund Handl. vol. viii (1897) 

 p. 2. 



3b2 



