LOWER HELDERBERG ROCKS. 
158 
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 upon 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 0. elegantula of the Niagara 
group is represented by 0 . subcarinata and 0 . perelegans; 0 . hybrida , by 
0. oblata and 0. discus, a smaller form; and O. punctostriata, 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 
Rhynchonellce. 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 
