Vol. 56.] THE LOWER LUDLOW FORMATION. 449 



splintery flags. Speaking generally, there is a gradual decrease of 

 argillaceous material as the beds are followed from base to summit 

 of the series (for detailed comparison see Table I, p. 448). 



With regard to the distribution of the Lower Ludlow graptolites, 

 it may be pointed out that some forms, as, for instance, Mono- 

 graptas Nilssoni and M. bohemicus, are very widely distributed over 

 Britain and Europe. Some appear to be associated only with 

 sediments of a particular lithological character, as, for example, 

 M. scanicus, which occurs in Scania and the southern and south- 

 eastern districts of Britain, but is rare or absent in the northern 

 and north-western areas. Some forms again seem to be quite local, 

 as, for instance, M. crinitus, which is practically confined to the 

 Long Mountain district. 



It is hardly surprising, therefore, to find that of the five graptolitic 

 zones which may be recognized in one or other of the several British 

 districts, only two, those of M. Nilssoni and M. leintwardinensis, 

 are common to them all. The zone marked by M. vulgaris, although 

 persistent in those areas where there is a little or no development 

 of purely calcareous sediments, is unknown in the Ludlow district. 

 The zone marked by M. scanicus is well shown in two districts, 

 namely, those of Ludlow and Builth (eastern area), but in other 

 districts it is either wanting in graptolites or else the zone-fossil is 

 absent. The zone of M. tumescens is confined to two areas, namely, 

 those of Ludlow and the south side of the Long Mountain, but in 

 these it occurs in great abundance. 



As respects the distribution of the graptolites in these several 

 ones, an examination of Table III (p. 450) shows clearly that the 

 great majority occur in association in the zones of M. Nilssoni and 

 M. scanicus, while all the other zones yield only a few species. 



Regarding the limits of the Lower Ludlow Shales, we have seen 

 that the boundaries based on purely lithological characters are 

 artificial, for the Wenlock and Aymestry Limestones are confined to 

 special areas. Any natural and universally applicable division must 

 be determined by palaeontological considerations. Such a division 

 necessitates, in my opinion, the inclusion of the Aymestry Limestone 

 in the Lower Ludlow formation. 



Bo detailed comparison with the Lower Ludlow deposits of Europe 

 is at present possible, as no zonal work has been done in these beds 

 outside Britain. A glance at Table II (facing p. 450), however, will 

 shAw that many of the species of Lower Ludlow graptolites found 

 in Britain occur in Scania, Bohemia, Germany, and France. One 

 awaits with interest the forthcoming section of Perner's 'Etudes 

 sur les Graptolites de Boheme, ' dealing with the zonal divisions of 

 the graptolitic rocks of Bohemia, the graptolites of which seem to 

 be so closely allied to those of Britain. 



