186 Mr. C. Lapworth on the Geological 



(Didymograplus sparsas, Hopk., and D. pennatulus, Hall) are 

 found in the lowest Arenig strata of St. David's, at the base 

 of the Ordovician of that region. Its most recent form hitherto 

 detected (D. superstes, Lapw.) is by no means uncommon in 

 the earlier beds of the Glenkiln (Llandeilo-Bala) shales of 

 South Scotland. 



Of this genus several well-marked subgroups are recog- 

 nizable, distinguishable by the general shape of the polypary ; 

 but our knowledge of their range is not as yet sufficient to 

 enable us to draw any reliable conclusions respecting their 

 individual existence in time or space. One group, however 

 — that of D. Murchisoni {geminus) — is remarkable, as its 

 forms are essentially characteristic of the great dark-shale 

 zone of the Arenig-Llandeilo, in which they are everywhere 

 the preponderating fossils. 



Tetragraptus. — The four-armed genus Tetragraptus , Salt., 

 stands next to Didymograptus in abundance of species and 

 individuals. Its range, however, is peculiarly circumscribed, 

 none of its forms occurring outside the limits of the lower or 

 typical zones of the Skiddaw -Arenig formation, which it 

 characterizes not only in Wales, but also in England, Scan- 

 dinavia, and North America. 



Complex Genera. — Of the more complex genera of the 

 Dichograptidee we recognize two artificial groups, viz. (a) 

 those in which the mode of origin of the branches is regularly 

 dichtomous, and (b) those in which it is lateral or irregular. 

 The regular genera are as yet exclusively Skiddaw- Arenig 

 forms. The irregular genera range from the Cambrian to 

 the middle of the Llandeilo. The former group claims the 

 highly complex genus Loganograplus, Hall, the horizontal 

 distribution of which is world-wide. To the latter group 

 belongs the most ancient genus of the Dichograptida3 — 

 the genus Bryograptus, Lapw., which is supposed to be 

 exclusively characteristic of the 0?emw-beds of the Upper 

 Cambrian. 



Family v. Phyllograptidse. 



I take this family next in order, from its undoubted rela- 

 tionship to the foregoing, and from its general similarity in 

 geological distribution. Its single genus appears to be re- 

 stricted exclusively to strata of so-called Arenig age. None 

 of its species are known -to occur above or below the provi- 

 sional limits assigned to this formation in any of the widely 

 separated regions where they have been recognized hitherto. 

 The genus culminates along the same general zone of the 

 typical Skiddaw- Arenig as its ally Tetragraptus ; but instead 



