414 



DEVONIAN PERIOD 



the early Devonian faunas of the northern latitudes of Europe and 

 America. 



The main features of the Helderberg fauna were great numbers 

 of mollusks and brachiopods, an erratic tendency of the trilobites, 



Fig. 363. HELDERBERGIAN FOSSILS: a, Poly par a lilasa (Hall), a fenestelloid 

 bryozoan representative of a group which was of great importance later; b, Michc- 

 linia lenticular is Hall, the earliest member of a genus of corals which became abun- 

 dant in later Devonian faunas ; c, Lepocrinitcs gebhardii Con. , one of the last represen- 

 tatives of the cystids. d-n, Brachiopods: d, Rensselceria cequiradiata (Con.), a 

 representative of a genus characteristic of the Lower Devonian; c, Spirifer macro- 

 pleurus (Con.), a species closely related to the Silurian species of the genus; /, 

 Strophonclla punctulifera (Con.); g, Schizophoria multistriata (Hall); h, Uncinuhts 

 mutabilis (Hall), a representative of a genus which had its greatest development in 

 the Helderbergian fauna; i, Gypidula galeata (Dal.), one of the most characteristic 

 species of the Lower Helderberg; j, Bilobites various (Con.), a type of orthid charac- 

 teristic of the Silurian and Helderbergian; k, Eatonia mcdialis (Van.), a representa- 

 tive of a genus most characteristic of the Lower Devonian; /, Rhipidometta oblala 

 (Hall); m, Leptcena rhomboidalis Wilck., a species which ranges from the Ordovician 

 to the Mississippian; n, Atrypina imbricata (Hall), a lingering Silurian type; o, 

 Actinopteria textilis (H.a.11) , a winged pelecypod of a type which had great expansion 

 in the Devonian; p, Platyccras gibbosum Hall, a capulid gastropod; q, Dicranurns 

 hamatus (Con.), a trilobite whose closest relative occurs in Barrande's Etage G, in 

 Bohemia; r, Phacops logani Hall, a representative of a genus of trilobites which had 

 its greatest development in the Devonian. 



