422 MISS E. M. E. WOOD ON THE LOWER LUDLOW [May I0,OO, 



It will be seen, therefore, that the typical Lower Ludlow fauna 

 is distinct from that of the typical Wenlock, yet so imperceptibly 

 does the one merge into the other that it is difficult to fix the exact 

 horizon of separation. At the horizon of the Monograptus-Nilssoni 

 zone one is undoubtedly in Ludlow ground, and the horizon of the 

 Cyrtograptus-Lundgreni zone is as certainly Wenlock. Between 

 these horizons intervenes the zone of M. vulgaris, sp. nov. ; this, 

 where well developed, consists of very thick, hard, calcareous, flaggy 

 shales, and generally makes a feature in the landscape. The palaeon- 

 tological character of this zone allies it more closely with the Ludlow 

 than with the Wenlock, and therefore we have regarded the ba|se 

 of the M -vulgaris zone as the dividing-line between 

 the Wenlock and the Ludlow formations. 



The determination of the upper boundary of the Lower Ludlow 

 is also a matter of some difficulty. ]S"o graptolites have been found 

 hitherto in the Upper Ludlow Beds of Britain. The majority of 

 the species die out in the Lower Ludlow Shales, but one species 

 characteristic of the highest zone of these beds, namely, M. leintwar- 

 dinensis, Hopk. certainly ranges up into the Aymestry Limestone. 

 So far, then, as the graptolitic evidence goes, the limit between 

 the Lower and Upper Ludlow Beds must be drawn at 

 the top of the Aymestry Limestone, for obviously it would 

 be unadvisable to draw the boundary in the middle of a graptolitic 

 zone. In those areas where the Aymestry Limestone is well developed, 

 as in the Ludlow district, or even where it is but slightly represented, 

 as in the Builth district, there is little difficulty in fixing this upper 

 boundary more or less exactly. But in those areas, such as the 

 Long Mount ain, where there is no calcareous representative of the 

 Aymestry Limestone, it becomes practically impossible to draw the 

 line between the Lower and Upper Ludlow on graptolitic evidence 

 alone, the graptolites dying out so gradually that one is unable to 

 determine where they cease altogether. In such cases the lithological 

 characters of the rocks must to a large extent supplement, or indeed 

 entirely replace, the palaeontological evidence. 



(2) The Zonal Divisions of the Lower Ludlow. 



The limited vertical range of the individual species of graptolites 

 in general, together with their wide distribution in space, make 

 them peculiarly suited for zonal fossils ; and most of the Lower 

 Palaeozoic rocks which contain graptolites, up to and including 

 the Wenlock Shale, have already been variously divided into zones, 

 each of which has a distinctive name. These zones have been 

 found to follow each other invariably in the same order both in 

 Britain and in Western Europe, and it was inferred long since by 

 graptolithologists, such as Mr. Hopkinson and Prof. Lapworth, that 

 detailed work on the graptolites of the Lower Ludlow Shales would 

 demonstrate a corresponding zonal distribution of the species in 

 these beds. 



In those districts which I have studied in detail, I have found 



