THE DEVONIAN PERIOD. 



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Fig. 206. — The ETelderbergian Fauna. Gastropoda: a, Platyceras spirale Hall; 

 b, Platyceras gibbosum Hall, two forms of the capulid shells which abound at some 

 localities in the Helderbergian and which have their great development in the 

 Devonian faunas. Pelecypoda: c, Actinopteria textiles (Hall), one of the winged 

 pelecypod shells which have so great an expansion in the Devonian faunas. 

 Brachiopoda: d, Spirifer macropleurus (Con.), a species which is more closely 

 related to the Silurian than to the Devonian members of the genus; e, Strophonella 

 punctulifera (Con.). The members of this genus are more characteristically 

 Silurian than Devonian although it persists to the Middle Devonian; /, Leptcena 

 rhomboidalis Wilck., a species which ranges from the Ordovician to the Missis- 

 sippian (see Fig. 163); g, Gypidula galeata (Dal.), one of the most character- 

 istic species of the Lower Helderbergian or Coeymans fauna; h, Rensseloeria 

 wquiradiata (Con.), a representative of a genus peculiarly characteristic of the 

 Lower Devonian; i, Uncinulus mutabilis (Hall), a representative of a genus of 

 rhynchonelloid shells which has its greatest development in the Helderbergian 

 faunas; /, Schizophoria multistriata (Hall), and k, Rhipidomella oblata (Hall), 

 shells representing two types of orthids having their greatest development in 

 the Devonian faunas; I, Dalmanella subcarinata (Hall), another orthid of a group 

 more strongly represented in earlier faunas; m, Eatonia medialis (Van.), a repre- 

 sentative of a genus most characteristic of the Lower Devonian faunas; n, 

 . Atrypina imbricata (Hall), a lingering Silurian type; o, Bilobites various (Con.), 

 a type of orthid characteristic of the Silurian and Helderbergian; p-q, Cyrtina 

 dalmani Hall, a member of a genus feebly represented in the Silurian but which 

 has a great development in the Devonian faunas. Bryozoa: r, Polypora lilcea 

 (Hall), one of the fenestelloid bryozoans, a group having a slight representa- 

 tion in earlier faunas, but attaining great prominence later. Corals: s, Michelinia 

 lenticularis Hall, the earliest member of a genus of corals which becomes abundant 

 in later Devonian faunas. Cystoidca: t, Lepocrinites gebhardii Con., one of the 

 last representatives of the cystids, a group of echinoderms having their greatest 

 expansion in Ordovician and Silurian times. Trilobita: u, Phacops logani Hall, 

 the genus Phacops, though present in the Silurian, has its great development in 

 the Devonian; v, Dicranurus hamatus (Con.), an acidaspid whose closest relative 

 occurs in Barrande's etage G, in Bohemia. (Weller.) 



