692 MISS E. M. B. WOOD ON THE [Nov. 1906, 



are recognizable: — The highest or the Twymyn Group consists 

 of pale mudstones, with occasional black shale-bands. It contains 

 two zones, namely those of Monograptus SedgwicJcii and CepJialo- 

 graptus cometa. It passes down with complete conformity into the 

 Dolgadfan Group (zones of Monograptus convolutus and M.Jvm- 

 briatus). the sediments of which are fairly similar to those of the 

 overlying group but contain. more numerous black bands. The 

 lowest beds yet detected in the district constitute the Fachdre 

 Group (zone of DimorpJwgraptus Swanstoni), and consist mainly 

 of sandy or calcareous mudstones with some gritty bands. 



(B) Comparisons with the Graptolitic Deposits of other 

 Areas. 



(a) Palaeontological. 



With the exception of two small brachiopods, an OrtJioceras, a 

 doubtful trilobite, and abundant worm-burrows, the only fossils 

 hitherto found in this Tarannon district are graptolites. These are, 

 however, numerous both as regards individuals and as regards 

 species. The identification of several forms has been rendered 

 difficult, partly on account of their imperfect preservation, and 

 partly from our lack of exact knowledge concerning many of the 

 species found at corresponding horizons in other parts of Britain. 

 Hence the comparison of the graptolites, based as it is on im- 

 perfect lists, is by no means so complete and satisfactory as it would 

 otherwise be. 



The accompanying Tables illustrate the geological range of the 

 various species of graptolites found in the Tarannon district and their 

 geographical distribution. In Table I the species are arranged in 

 the first column in alphabetical order ; in Table II they are arranged 

 so as to show, as far as possible, their order of appearance in the 

 Tarannon country. The relative abundance of the species is also 

 denoted by letters, as without this it is almost impossible to form 

 a correct idea of the distinctiveness of the local zones. 



Examining these lists generally, we can have no hesitation in 

 paralleling the Llandovery Series of the Tarannon district with the 

 Birkhill Beds of the South of Scotland, the Tarannon Series with 

 the Gala Group, and the Wenlock Series with the Bicearton Beds. 



Before discussing the question of the fauna of the Tarannon 

 Series, we may briefly consider that of the underlying Llandovery 

 Series. Out of the total number of 39 species found in this series, 

 29 are known to occur in the Birkhill Shales, and the remaining 

 LO are forms which have been described since the Birkhill Shales 

 were originally zoned by Prof. Lapworth. 



Considering first the FachdreBeds, we find it possible to institute 

 a close comparison between their fauna and that of the highest band 

 of the zone of Diplograptus vesiculosus in the Moffat District. This 

 band is there, here, and elsewhere, characterized by the presence of 

 species of Dimorphograptus and by the occurrence of the first known 

 Monograptid, namely Monograptus tenuis. 



