PLICATIONS OF PARASTROPHIA HEMIFLICATA, HALL. 
137 
size of the specimens from the Prasopora beds. The additional 
plications on the sides, then, must be due to the stage of develop- 
ment of the shells of that horizon not to the growth of the 
individual shell. 
There seems, however, to be no definite ratio between the 
number of plications in the sinus and that on the sides. Many 
of the shells are so worn on the sides that there is frequently 
just the indication of a plication on the edge, its extent being 
obliterated. It is to be noted, however, that those plications on 
the sides which are nearest the sinus form first and are usually 
by far the strongest, as well as the earliest to appear . 
