SILURIAN FOSSILS 



63 



14. 



a ) 





b) 



15. 



a) 





b) 



16. 



a) 





b) 



17. 



a) 





b) 



18. 



a) 





b) 



19. 



a) 





b) 



20. 



a) 





b) 



21. 



a) 





b) 



22. 



a) 





b) 



Outline of shell decidedly triangular Rhynchotreta 



Outline of shell more or less rounded 15 



Beaks forming a distinct point Stegerhynchus 



Beaks rounded 16 



Shell wider than long Anastrophia 



Shell length and height about equal 17 



Shell very fat, globe -shaped Uncinulus 



Shell not globe-shaped, only moderately thick Homoeospira 



Hinge line more than half the width of the shell Dolerorthis 



Hinge line less than half the width of the shell Conchidium 



Hinge line more than half the width of the shell. 

 Hinge line less than half the width of the shell. . 



20 

 22 



With sinus and fold 21 



Without sinus and fold Brachyprion, Fardenia, Protomegastrophia 



and Strophonella 



Sinus and fold inconspicuous Parmorthis 



Sinus and fold strong Eospirifer 



Valves of almost, equal thickness Rhipidomella 



Valves of unequal thickness Atrypa 



Trimerella (fig. 99) is a rather large brachiopod, generally 

 found as internal molds which are recognizable by the four stout 

 prongs, two on each side of the valve. Two species are com- 

 mon in the Guelph of Ohio. 



Whitfieldella can be recognized by its smooth shell which 

 is longer than wide, and its pointed beak. Two species of Whit - 

 fieldella are common in the Niagaran. Older publications men- 

 tion a third common species of Whitfieldella which is now placed 

 in the next genus. 



Cryptothyrella cylindrica ( Whitfieldella in older publica- 

 tions) is like that genus but the beak is fat and round, not pointed 

 (fig. 100). Some of the shells of this species are almost cylin- 

 drical in cross-section. The species is common in the Niagaran. 



Ftg. 99 



