July 11 tli, 1914. 
, Canada 
Geological Survey 
Museum Bulletin No. 2. 
GEOLOGICAL SERIES, No. 18. 
VII. — A Preliminary Study of the Variations of the Plications 
of Parastrophia hemiplicata, Hall. 
By Alice E. Wilson. 
Parastrophia hemiplicata Hall 1 is found in great abundance 
in certain zones of the Trenton limestone. From several of 
these zones a considerable number of specimens were collected, 
the adults of which showed a great variation in the number of 
plications on the valves. A few adult specimens have a simple 
sinus with no plications, while the majority have from one to 
five in the sinus with the corresponding number on the fold. 
A study of them was undertaken to ascertain if possible whether 
all of the forms really belonged to one highly variable species, 
and, if so, whether any reason for their great variability could 
be found. For the purpose of study the shells were 
provisionally assorted into groups according to the number of 
plications in the sinus. Each variation has been considered as a 
group — that is the no-plication group, the one-plication group, 
etc. These groups so merge into one another that they can 
hardly be regarded as separate species, and many of the individual 
shells of several plications in their growth pass through some of 
the stages of those groups which have fewer plications. 
The shells used in this study come from three divisions of 
the Trenton limestone — the base at Indian Lorette, Quebec, 
the Prasopora beds in the vicinity of Ottawa and Hull, and a 
higher horizon represented by the specimens from Fifth 
1 Pal. N. Y. vol. I, p. 144, 1847. 
131 
