Vol. 62.] TARANNON SERIES OE TARANNON. 693 



The Dolgadfan Beds are represented in the Tarannon District by- 

 two zones, namely, those of Monograptus fimbriates and M. con- 

 volutus. These correspond collectively to Prof. Lapworth's single 

 zone of M. gregarius of the Birkhill Shales. That zone contains 

 a rich and varied fauna, and subsequent work l has shown the 

 possibility of dividing it into at least three distinct sub-zones, of 

 which M. Jimbriatus is practically the lowest. 



The assemblage of species met with in the Dolgadfan zone of 

 M. convolutus corresponds to that of the upper part of Prof. Lap- 

 worth's zone of M. gregarius. This zone has been detected at two 

 different localities in the Twymyn Valley near Dolgadfan, and the 

 species vary considerably at the two spots. In its lowest band, the 

 zone-fossil itself is rare and M. leptotheca and M. cf. decipiens are 

 the abundant forms ; while in its upper band these two species 

 appear to be absent, or at any rate are very rare, and a form which 

 finds its nearest ally in M. communis is the most prevalent species. 



The two zones of the Twymyn Beds, namely those of Cephalo- 

 graptus cometa and Monograptus Sedgwickii, can be exactly paralleled 

 with the corresponding zones in the Upper Birkhill Shales of the 

 South of Scotland. The zone-fossil of the ' cometa '-band has not 

 been detected as yet in the Tarannon district, but it is always a rare 

 form. The graptolites found in the local zone of M. Sedgwickii are 

 typical of that zone wherever it has been recognized. In its lowest 

 beds, M. Sedgwickii and M. discretus occur almost to the exclusion 

 of other species ; while in the upper part new species characteristic 

 of still higher horizons begin to make their appearance. 



Thus, in the Llandovery Series of the Tarannon district, which 

 underlies conformably the Tarannon Series, we have the undoubted 

 representatives of all the Lower, Middle, and Upper Birkhill zones 

 with the exception of the lowest (Diplograptus acuminatum) and the 

 highest (Rastrites maocimus). The former is not laid bare in this 

 district, and the latter will be referred to subsequently. 



Turning now to the Tarannon Series, we find that in this district 

 it has been possible to distinguish four local zones, namely, those of 

 Monograptus turriculatus, M. crispus, M. griestonensis, and M. crenu- 

 latus. Some of these have been already recognized elsewhere (p. 696); 

 and as our knowledge of the distribution of the graptolites increases, 

 it is possible that these zones may be found to have more than a 

 local application. 



A close comparison between the Tarannon Series as a whole, and 

 the Lower and Upper Gala Beds of the South of Scotland can be 

 instituted, if the zone of Rastrites maximus is included. It is true 

 that out of the 38 species found in this district, only 18 have been 

 recorded from the South of Scotland ; but of this total of 38 no less 

 than 13 are forms unknown when the Scottish beds were zoned, 

 and the comparison would be far closer if revised lists of the Scottish 

 graptolites were taken instead of the published lists. 



1 J. E. Marr & H. A. Nicholson, Quart. Journ. Geol. Soc. vol. xliv (1888) 

 p. 654; and H. Lapworth, Quart. Journ. Geol. Soc. vol. lvi (1900) p. 67. 



