364 Mr. C. Lapworth on the Geological 



This fauna reminds us at once of that of the Arenig for- 

 mation in the extraordinary predominance of a single family 

 of the Rhabdophora. The Monograptidse, of which a few 

 scattered examples are found for the first time in some of the 

 lowest Llandovery beds, multiply so rapidly that long before 

 we reach the middle beds of the Valentian formation they have 

 become the dominant family. For a time the Diplograptidae 

 hold their own, as if contesting every horizon ; but at the 

 very commencement of the Tarannon period the issue is prac- 

 tically decided. One by one the species of the Diplograptidae 

 sink out of sight ; and when the true Wenlock is reached the 

 last has vanished. A Retiolite form occurs alone for some 

 time longer ; but in the Lower Ludlow the Monograptus 

 reigns unchallenged, the solitary survivor of its race. 



As in the case of the Glenkiln and Caradoc formations, we 

 turn to the south of Scotland for our typical Graptolite-bearing 

 rocks of Valentian age, where they compose the Birkhill 

 shales and the overlying Gala group. These strata have been 

 so minutely searched for Rhabdophora that there seems to be 

 an uninterrupted zoological gradation from the base to the 

 summit of the formation. In the entire series we seem at 

 present to recognize five subgroups of tolerably equal syste- 

 matic importance — the (1) Lower, (2) Middle, and (3) Upper 

 Birkhill shales, and (4) (5) the Gala and Grieston groups. Of 

 these, nos. 1, 2, and 3 are possibly included in the so-called 

 Lower Llandovery formation of South Wales, while the fourth 

 and fifth correspond to the Upper Llandovery and Tarannon. 



(A) Lower Birkhill. — This includes the so-called zones of 

 Diplograptus acuminatus, Nich., and D. vesiculosus of Dobbs 

 Linn &c, and contains but few Graptolites in addition to 

 those which give their names to the zones. M. tenuis. Port- 

 lock, and M. attenuatus, Hopk., are its only Monograptidae, 

 and are the first species of this family hitherto detected. The 

 remaining fossils are Diplograptidae of extended range. 



(B) Middle Birkhill. — This division, which embraces the 

 thick zone of Monograptus gregarius, Lapw., is most prolific 

 in Rhabdophora, and is recognizable not only in the Moffat 

 area, but also in Girvan, Ireland, and Sweden. In this zone 

 the Diplograptidae and Monograptidae are tolerably equal in 

 species and individuals. The species D. vesiculosus, Nich., 

 and 1). physophora, Nich., make here their final appearance. 

 The genus Rastrites (Barr.) occurs for the first time ; and the 

 single species Rastrites per 'egrinus is remarkably prolific. Of 

 the genus Monograptus species are abundant, but few are 

 peculiar. Among the latter is the transitional form M. trian- 

 gulatus, Harkn. 



