284 On the Distribution of the Rhabdophora. 



Llandeih-Bala or Glenkiln Shales.' — In these unplaced 

 beds, which possibly include parts of the higher Llandeilo and 

 Lower Bala, as these formations are defined at present, the col- 

 lective fauna is essentially of the Bala type. A single patuli- 

 form species of Didymogrcuptus (perhaps two) is all that 

 remains to represent the hosts of Dichograptidas of the Arenig 

 age ; and this becomes extinct before we reach the highest beds 

 of the subformation. The characteristic fossil of the Glenkiln 

 shales is Citnograptus gracilis, Hall, of which it may be re- 

 garded as the special zone. This well-marked fossil occurs 

 upon the Glenkiln horizon in Britain, Scandinavia, America, 

 and Australia ; it is accompanied by some few allied species 

 of the same genus, and more markedly by crowds of Dicrano- 

 graptidaa. Dicranograptus ramosus, a fossil of the overlying 

 Bala, accompanies Camograptus in its range from Britain to 

 the Antipodes, with some closely allied transitional forms of 

 the type of Dicranograptus formosus, Hopk. One of the most 

 conspicuous fossils of the zone is Dicellograptus sextans, which 

 occurs in extraordinary abundance in the lower beds, and 

 probably equals Coenograptus in its extended geographical 

 range. In Scotland and Wales, and in the Norman's-Kiln 

 beds of North-east America, it is also associated with some 

 closely allied forms, representative of Dicellograp>tus divaricatus, 

 Hall. 



Diplograptidae are everywhere abundant. Diplograptus 

 rugosus, Emm., is one of the commonest species; and the 

 beautiful form D. euglyphus (? D. putillus, Hall) is locally 

 prolific. D.foliaceus, Murch., is always present, but is less 

 abundant than in the succeeding Bala-Caradoc beds. Crypto- 

 graphs tricornis, Carr., is frequently seen in several varie- 

 ties, and the peculiar form D. perexcavatus, Lapw. But the 

 species most commonly met with are undoubtedly those of the 

 genus Climacograptus. C. ccelatus, Lapw., C. Scharenbergi, 

 Lapw., and C. bicorm's, Hall, are all present, the latter in 

 many peculiar subspecific forms. 



Of the Lasiograptidse there are several species ; and the 

 members of this family are more abundant in this zone than 

 upon any other horizon hitherto detected in Britain. Lasio- 

 graptus costatus, Lapw., a possible variety of L. Harknessi, 

 Nich., is abundant in the south of Scotland and in the north 

 of Ireland. The provisional genus Hallograptus of Carru- 

 thers, with its scopulate reproductive appendages, is as yet 

 wholly confined to this zone, where it has been met with in 

 Scotland, Ireland, and North America. Glossograptus ; Emm., 

 here presents us with some of its latest species ; their highly 



