576 



GEOLOGY 



Fig. 414. The Helderbergian Fauna. a, Polypora lilcea (Hall), a 

 fenestelloid bryozoan representative of a group which was of great 

 importance later; 6, Michelinia lenticularis Hall, the earliest member of 

 a genus of corals which became abundant in later Devonian faunas; 

 c, Lepocrinites gebhardii Con., one of the last representatives of the cys- 

 tids. d-n, brachiopods: d, Rensselceria cequiradiata (Con.), a represen- 

 tative of a genus characteristic of the Lower Devonian; e, Spirifer mac- 

 ropleurus (Con.), a species closely related to the Silurian species of 

 the genus; /, Strophonella punctulifera (Con.); g, Schizophoria mitlli- 

 striala (Hall); h, Uncinulus mutabilis (Hall), a representative of a genus 

 which had its greatest development in the Helderbergian fauna; /, 

 Gypidula galeata (Dal.), one of the most characteristic species of (he 

 Lower Helderberg; /, Bilobites various (Con.), a type of orthid eharacter- 

 istic of the Silurian and Helderbergian; k, Eatonia medialis (Van.), a 

 representative of a genus most characteristic of the Lower Devonian; /, 

 Rhipidomella oblata (Hall); m, Leptcena rhombnfihilis Wilck., a species 

 which ranges from the Ordovician to the Mississippian; //, Atri//>in<i ////- 

 bracata (Hall), a lingering Silurian type; o, Actin<>/>!< mi i<.rt/'/,s (Hall), a 

 winged pelecypod of a type which had great expansion in the Devonian; 

 p, Platyceras gibbosum Hall, a capulid gastropod; q, Dicra mints h<un<itnx 

 (Con.), a trilobite whose closest relative occurs in Barrande's I J 

 in Bohemia; r, Phacops logani Hall, a representative of a genus of trilo- 

 bites which had its greatest development in the Devonian. 



