278 Mr. C. Lapworth on the Geological 



wards Ccenograptus, one of the most striking forms of the 

 distinct family of the Leptograptidge. 



From the base to the summit of the Arenig formation the 

 Dichograptidse are accompanied by the rarer forms of the most 

 intimately allied family of the Phyllograptidte. The single 

 genus of which this family is at present composed has not 

 hitherto been met with in true Cambrian rocks, and is un- 

 known above the lower limit of the Llandeilo formation. Like 

 the Dichograptidee, its species are typical of the Arenig rocks 

 from Scandinavia to the Antipodes. We know too little of 

 the beautiful forms of this family to note any general change 

 in their superficial features during this extended period. As 

 yet, indeed, we have not collected sufficient material for the 

 complete study of a single British species. 



As a general rule, species belonging to the remaining 

 families of the Diprionida are, in the Arenig, the rarest of 

 fossils. Locally, however, an occasional stratum is met with 

 in which they are tolerably abundant. Three distinct families, 

 however, are already recognizable, all making their first 

 known appearance almost simultaneously near the boundary 

 line between the Lower and Middle Arenig rocks. The 

 Diplograptidae are represented by the whole of the three com- 

 ponent genera ; but the species are all somewhat generalized 

 in character, and the most generalized genus, Gryptograptus^ 

 claims, as might have been expected, the majority of the 

 known forms. The intermediary and provisional family of 

 the Lasiograptidre is represented by the bizarre genera Glosso- 

 graptus and Eetiograptus of Hall. The latter, which is as 

 yet confined to the Lower Arenig of Canada and Australia, is 

 also the most generalized genus of its family, combining 

 structural features subsequently found separate in Glosso- 

 graptus and Lasiograptus. The Retiolitidas are represented by 

 a single genus only, Tetragraptus of Nicholson, which seems 

 likewise of a transitional character, leading through the later 

 genus Gymnograptus of Tullberg into the family of the 

 Lasiograptidse. 



Of the Monoprionida outside the dominant family of the 

 Dichograptida? few forms are known, a single British ex- 

 ample of the Dicranograptidse from the Upper Arenig of 

 Abereiddy Bay and two species of the family of the Lepto- 

 graptidse from the corresponding strata of the Upper Skiddaw 

 group are all that have hitherto been published from these 

 Arenig rocks. 



Lower Arenig (of Hicks). — The only locality where grap- 

 tolitiferous strata near the base of the Arenig formation 

 have been carefully examined in Britain is at Whitesand 



