Distribution of the Rhabdophora. 107 



Lower Ordovician. 



2. Zone of Tetragraptus (bryonoides, Hall). — This is tlie 

 typical Quebec or Skiddaw Graptolite zone. It is strikingly 

 individualized by the exclusive possession of all the known 

 forms of the genus Tetragraptus, Hall. The genera Logano- 

 graptus, Hall, Clonograptus, H., Schizograptus, Nich., and 

 Dichograptus, Salt., are all probably peculiar to this zone, 

 as are also the species Didymograptus extensus, Hall, D.pen- 

 natulus, Hall, and the only known examples of Retiograptus, 

 Hall. This zone is recognizable at St. David's, Shelve, 

 Skiddaw, Norway, Scania, and in North America and Aus- 

 tralia, everywhere distinguishable by the same group of forms. 



3. Zone of Didymograptus bifidus, Hall. — This zone finds 

 its typical representative in the Upper Skiddaw beds of the 

 Lake District and in the " Phyllograptus beds " of Scania. 

 It is most especially marked by the presence of Phyllograptus ) 

 in association with geminiform Dichograpti of the group 

 typified by D. bifidus, Hall. From the zone below it is dis- 

 tinguishable by the extreme rarity of compound Dichograptidas. 

 Its peculiar fossils are D. bifidus, Hall, D. minutus, Tullberg, 

 and some forms of Diplograpta, such as Climacograptus con- 



fertus, Lapw. Many of its commonest forms in Britain appear 

 to be survivals of those of the underlying zone, such as Phyllo* 

 graptus angustifolius, H., P. typus, Hall, Didymograptus 

 patulus, Hall, D. affinis, Nich., D. Nicholsoni, Lapw.) &c. 

 The zone has been identified at Llavirn near St. David's, at 

 Tyobry, at Shelve, in Cumberland, and in Scania and Da- 

 larne. 



4. Zone of Didymograptus Murchisoni, Beck (geminus, 

 His.). — This is the typical Upper-Llandeilo Graptolitic zone 

 of Murchison, but both physically and palseontologically it 

 appears to be most distinctly allied to the foregoing Upper 

 Arenig zone. It is characterized mainly by the exclusive 

 presence of the form which gives it its name, by the total 

 absence of the genus Phyllograptus, and by a few distinctive 

 Diplograpta, of which the best-known is Climacograptus 

 ccelatus, Lapw. 



The zone is recognizable by position, mineralogical cha- 

 racter, and fossils in Britain at Llandeilo, Builth, Shelve, 

 Abereiddy, Pont Seiont, in Scandinavia near Christiania, in 

 Scania, and in Brittany and Portugal. 



Tipper Ordovician. 



In Wales the Upper Ordovician is separated from the 

 Lower by a series of grits and trap-rocks with possible tincon- 



