& 



Distribution of the RhabdopJiora. 189 



Family viii. Eetiolitidse. 



Of all the families of the Rhabdophora this has the greatest 

 vertical extension. Its known range extends from the base 

 of the Ordovician to the middle of the Silurian proper. It 

 must, however, like the previous family, be regarded as being 

 essentially an artificial group, including genera of very un- 

 certain zoological relationships. 



Trigonograptus, Nich. — The oldest of the genera provi- 

 sionally referred to this family is the genus Trigonograptus, 

 Nich., with very thin continuous or punctate epiderm and 

 faintly markecl-off hydrothecas. It is purely an Arenig 

 genus. 



Gymnograptus, Tullberg. — This is certainly most intimately 

 allied to the foregoing, standing in some respects between it 

 and the typical Lasiograpticla3. It is abundant in Scania, in 

 the Llandeilo beds, but has not hitherto been recognized in 

 Britain. 



Clathrograptus, Lapw. — Is one of the rarest of fossils in 

 the Llandeilo-Bala (Glenkiln) of Scotland and New York. 

 Its relationship to the strange complex form Retiograptus 

 eucJiaris, Hall (Grapt. Quebec Group, pi. xiv. fig. 9), is pro- 

 bable, but uncertain. 



Retiolites. — The genus Retiolites proper (Gladiolites or 

 Gladiograptus) ,with its reticulate periderm and faintly -marked 

 thecal walls, ranges from the Lower Bala into the Upper 

 Wenlock. 



The only known Ordovician example of the genus {Retio- 

 lites (Neurograptus) Jibratus, Lapw.) is remarkable for its 

 strong central virgula. Its gonosome is furnished with lateral 

 reproductive appendages (scopulEe), similar to those of Hallo- 

 graptuSj Carr. 



Retiolites Geinitzianus is the most widely distributed species 

 of the family. In Britain it ranges from the base of the 

 Tarannon to the higherWenlock, and is a common and charac- 

 teristic fossil in the corresponding strata in Scandinavia and 

 Central and Southern Europe. 



Part IV. Conclusion. 



We have now completed our survey of the available data 

 bearing upon the distribution of the Rhabdophora in space 

 and time, and of the more important conclusions to which 

 they appear to point. It only remains, finally, to indicate 

 the chief propositions which these results seem to place outside 

 the pale of future controversy, or render so highly probable 

 that they may be provisionally accepted as true. 



Ann. & Mag. N. Hist. Ser. 5. Vol vi. 14 



