282 On the Distribution of the Rhabdophora. 



together with new and very remarkable forms of Gryptograptus 

 and Glossograptus. In many respects this black-shale zone 

 is most naturally regarded as the upward extension of the 

 deep-water sediments of the Upper Arenig ; and we may even- 

 tually be forced to unite it more closely with that subforma- 

 tion in our improved schemes of geological classification. 



Middle Llandeilo. — In the coarse grits, sandstones, and 

 felspathic ashes that together make up the greater part of the 

 succeeding Middle Llandeilo subformation, Didymograptus 

 Murchisoni and some of its varieties are still present, but are 

 excessively rare. Of other forms few are known ; Diplo- 

 graptus foliaceus, Murch., and Climacograptus Scharenbergi, 

 Lapw., are all that have been actually recognized in Britain ; 

 but if the Glossograptus zone of the Scanian Middle Grap- 

 tolite schists admits of approximate parallelism with this 

 subformation, there fall to be added many additional species, 

 of which the commonest Swedish forms are allies of the pro- 

 lific Glossograptus Hincksi, Hopk., and Diplograptus rugosus 

 of the later Scottish Glenkiln formation. 



Upper Llandeilo. — The strata that lie between the typical 

 Llandeilo Limestone of South Wales and the Bala Limestone 

 of the North are not yet sufficiently studied to allow us to fix 

 the line of demarcation between the two formations to which 

 these calcareous zones give their titles. As we have already 

 pointed out, the lower strata are undoubtedly of Llandeilo age ; 

 but the larger mass of the beds lies well within the limits of 

 Sedgwick's Bala formation. Before we reach the limestones 

 and calcareous shales with Ogygia Buchi that lead up into the 

 Upper Llandeilo formation, all the geminiform Didymograpjti 

 so characteristic of the underlying beds have totally disap- 

 peared ; and I know as yet of no forms of Didymograptus 

 that have hitherto been collected in the Upper Llandeilo of 

 Wales. They must, however, be locally present ; for patuli- 

 form species occur in the higher Glenkiln shales. Diplo- 

 graptij however, are abundant, especially forms of the type of 

 Cryptoqraptus tricornis, Carr., and Climacograptus Scharen- 

 bergi, Lapw. Here also we meet for the first time with the 

 elegantly symmetrical genus Ccenograptus of Hall, so charac- 

 teristic of the Glenkiln shales, and its constant and highly 

 prolific though somewhat diminutive associate the species Dicel- 

 lograptus sextans of Hall, which in the Glenkiln period is as 

 numerically abundant as Didymograptus Murchisoni in the 

 Upper Arenig. 



