Distribution of the Ilhabdopliora. 201 



by the total absence of Diplograptidee, and by the presence of 

 many Wenlock forms, such as Monograptus vomerinus, M. 

 riccartonensis, &c. Strata with a similar transitional fauna 

 occur in Girvan, and form the earlier zones of Tullberg's 

 Retiolites Skiffar in Scania (Tullberg, Geol. For. Forh. 1880. 

 N. 59, B. 5. N. 3). 



Wenlock- Ludlow Series. 



17. Zone of Cyrtograptus Murchisoni, Carr. — The base of 

 the Wenlock series of Builth is formed by a highly fossilife- 

 rous series of shales crowded with G. Murchisoni^ M. vome- 

 7-inus, Nich., and a few survivals of the Grieston fauna. The 

 same fossiliferous zone has been met with in Shropshire, in 

 the valley of the Dee, North Wales, in Denbighshire, and 

 more doubtfully in the Lake District. It is well developed 

 in the succession at Rostanga in Scania, and is present also 

 in Bohemia. 



18. Zone of Cyrtograptus Linnarssoni, Lapiv. — The Wen- 

 lock shales of Shropshire have not yet been minutely sepa- 

 rated into their natural divisions ; but two fairly distinct zones 

 are already recognizable in their British or foreign equiva- 

 lents. To the lowest of these zones belong the strata near 

 Builth which succeed the C. -Murchisoni beds, and afford 

 C. Linnarssoni, Lapw., and a few other peculiar forms. It 

 is possible that the G.-Carruthersi bands of the Riccarton 

 beds of South Scotland belong to this general horizon ; but as 

 yet the typical Cyrtograptus has not been detected within 

 them. The middle beds of the Cyrtograptus Skiffar of Scania 

 have a corresponding Graptolite fauna. 



19. Zone of Monograptus testis, Barr. — In the highly pro- 

 lific graptolitiferous rocks of the south of Sweden the highest 

 zone that can with certainty be assigned to the equivalents of 

 the British Wenlock shales is characterized by the beautiful 

 form Monograptus testis, Barr. Its commoner associates are 

 abundant forms in the Wenlock of Britain ; but the typical 

 fossil itself has not hitherto been detected here. The zone is 

 recognizable in Bohemia, Thuringia, and France. 



20. Zone ofMonograptus Nilssoni, Barr. — The highest and 

 most important graptolitic zone of the Wenlock-Ludlow for- 

 mation is that which lies between the Wenlock and Aymestry 

 Limestones of Siluria and forms the Lower Ludlow shales 

 of Murchison. Its beds are of great thickness both in Britain 

 and Scandinavia, and will probably in the future be found 

 divisible into several distinct zones. This is shown by its 

 collective fauna, which is specifically very distinct from that 

 of the Wenlock shales. The most prolific form of the zone 



