LOWER HELDERBERG ROCKS. 1S3 



BRACHIOPODA OF THE LOWER HELDERBERG GROUP. 



The general resemblance of the Brachiopoda of this period to those of 

 the Niagara group is at once observed ujjon comparing collections from 

 the two groups : indeed so similar are many of the species of the Lower 

 Helderberg rocks to those of the Niagara group, that they are regarded 

 by many palaeontologists as identical. A careful comparison, however, 

 leaves much doubt whether even a single species has been found which 

 is common to the two periods. Almost every species of the Niagara group 

 is represented in the Lower Helderberg, not only by a similar species, in 

 general terms, but in the form, sculpture or surface markings, and ex- 

 ternal aspects ; and often to a great extent in internal characters, they 

 approach each other very nearly ; and not only is this so, but often we 

 find in the Lower Helderberg two or more analogues of a single Niagara 

 species. This is true of the Orthis, where O. elegantula of the Niagara 

 group is represented by O. subcarinata and O. perelegans ; 0. hybrida, by 

 0. oblata and O. discus, a smaller form ; and O. pundostriata, by 0. 

 tubulostriata. Spirifer niagarensis is represented by S. macropleura ; S. 

 sulcata, by S. perlamellosa ; S. crispus, by S. cycloptera ; and Strophomena 

 subplana, by S. woolworthana. The same comparisons may be made of the 

 Rhynchonella. Pentamerus fornicatus of the Clinton group is represented 

 by P. galeatus ; and P. ( Atrypa ) interplicata, a true Pentamerus, is re- 

 presented in the Lower Helderberg by P. verneuili. 



I might go much farther in these comparisons, showing how very 

 complete is the representation of the Niagara brachiopod fauna in the 

 Lower Helderberg group, and particularly in the shaly limestone, where 

 the physical conditions are so similar to those of the shales of the Niagara 

 group. 



Indeed were we to select these analogous forms alone, and present them 

 side by side with the brachiopods of the Niagara group, an experienced 

 palaeontologist might well hesitate in regarding them as distinct species, 

 [ Paleontology III.] 20 



