Distribution of the Rhabdophora. 281 



The collective Graptolite fauna of the typical Llandeilo 

 beds is essentially of a transitional character. In the true 

 Arenig strata, as we have already shown, the feathery Dicho- 

 graptidge and Phyllograptidse preponderate to such an extent 

 that the species of the few additional families represented 

 occur only as sparsely scattered specimens among the hosts 

 of individuals of Didymograptus and its allies. In the typical 

 Bala or Caradoc formation, as will be shown in the sequel, 

 these old Arenig families have utterly vanished, and the faces 

 of the graptolitiferous laminse are now crowded with multi- 

 tudes of Dicranograptidee and Diplograptidee. The Llandeilo 

 formation, as geologists provisionally define it at present, 

 combines in its collective fauna both the Arenig and Bala 

 types, and shows the gradual passage of the one into the 

 other. Phyllograptus is absent throughout ; but in the Lower 

 Llandeilo Didymograptus is as densely abundant as in the 

 Upper Arenig ; while Diplograptidge and Dicranograptidae are 

 very rare or only locally prolific. In the Upper-Llandeilo 

 and the transitional Llandeilo-Bala or Glenkiln strata, on the 

 other hand, a Dichograptid is the rarest of fossils, while the 

 Dicranograptidee and Diplograptidge occur in countless multi- 

 tudes. 



Loiver Llandeilo. — An occasional example of an irregularly 

 compound genus of the Dichograptidee has been met with in 

 the lowest zone of the Llandeilo, as near Llan Mill &c, where 

 I detected forms allied to Tetragraptus and Goniograptus, 

 M'Coy ; but, as in the immediately underlying beds of the 

 Upper Arenig, the most prolific genus in the Lower Llandeilo 

 is emphatically Didymograptus, M'Coy. Species with parallel 

 and with widely divergent arms are present; but both in 

 Britain and Scandinavia the " geminiform " species of the 

 type of Didymograptus MurcMsoni, Beck, occur in abundance 

 everywhere, while the " patuliform " species of the type of 

 D. patuluSj Hall, are only locally present. 



The black-shale beds composing the so-called Lower Llan- 

 deilo of Hicks may almost be denominated the " Zone of 

 Didymograptus Murchisoni, Beck." This beautiful fossil 

 abounds in the dark schists of the Lowest Division of the 

 Llandeilo of Abereiddy Bay, near Whitland and Llan Mill, at 

 Llandeilo, at Builth, and in the mining area of Shelve. It 

 reappears in like abundance in the " Didymograptus geminus 

 beds " of Scania, in Southern Sweden. In all these localities 

 it is accompanied by many of the Upper Arenig species enu- 

 merated in the previous section, or by very closely allied forms, 

 including representatives of Diplograptus dentatus?, Brongn., 

 Climacograptus ccelatus, Lapw.j and C. confertus } Lapw., 



