Vol. 56.] 



FORMATION AND ITS GRAPTOLITE-FAUNA. 



421 



(1) The Lower and Upper Limits of the 

 Lower Ludlow Formation. 



The Silurian system of Murchison in the typical area of Shropshire 

 and Herefordshire consists mainly of a great thickness of mud- 

 stones, of which the Wenlock Shales and Lower Ludlow Shales 

 form part. Murchison drew the line of division between the Wen- 

 lock and Ludlow formations at the Wenlock Limestone, but admitted 

 that the ' Lower Ludlow was simply an upward prolongation of the 

 Wenlock Shale,' and that in some districts ' it was impracticable to 

 endeavour to separate them.' Prof. Lapworth suggested that * such 

 a division was made probably ' less from a palaeontological than 

 from an aesthetic point of view, and mainly for the sake of physical 

 symmetry ' ; and urged that the Lower Ludlow was best united 

 with the Wenlock to form the middle division (Salopian) of the 

 Silurian system, the natural horizon of demarcation above it running 

 ' generally along the line of the Aymestry Limestone.' 



My study of the graptolitic fauna of the Lower Ludlow rocks 

 entirely bears out the views of both these authorities. In those 

 areas where there is a lithologicai transition between the Wenlock 

 and Ludlow Beds there is also a palaeontological transition, and the 

 one group passes gradually into the other. 



The Lower Ludlow graptolite-fauna, although having distinctive 

 characters of its own, yet possesses many marked affinities with 

 that of the Wenlock Shales : and, as I shall point out in the second 

 part of this paper, most of the groups of graptolites characteristic 

 of the Wenlock Beds find their representatives in the Ludlow. 



Considered as a whole, however, the graptolite-fauna of the 

 Lower Ludlow is sufficiently distinct from that of the Wenlock to 

 admit of the two formations being separated one from the other, and 

 such a palaeontological line of division is of considerable strati- 

 graphical and practical value. In ascertaining the most natural 

 horizon for this zonal line, I have worked in company with my 

 friend Miss G. L. Elles, and the following is the generalized result 

 of our joint observations: — The Wenlock Shales are everywhere 

 characterized by the presence of Cyrtograptus and by the Flemingii- 

 type of Monograptus. In the Lower Ludlow Shales neither of 

 these occurs, but instead the colonus- and spinose forms of Mono- 

 graptus, such as M. chimcera, are abundant. This may be more 

 readily shown in the following tabular comparison : — 



Wenlock Beds. 



Presence of Cyrtograptus. 



Presence of the Flemingii-tyipe of 

 Monograptus. 



Absence of the colonus-tyipe of Mono- 

 graptus. 



Absence of spinose forms of Mono- 

 graptus, such as M. chimcera, etc. 



Lower Ludlow Beds. 



Absence of Cyrtograptus. 



Absence of the Flemingii-type of 

 Monograptus. 



Presence of the colonus - type of 

 Monograptus. 



Presence of the spinose forms of Mono- 

 graptus, such as M. chimcsra, etc. 



1 ' Geological Distribution of the Rhabdophora,' Ann. & Mag. Nat. Hist. 

 «er. 5, vol. v (1880) p. 48. 



