PLICATIONS OF PARASTROPHIA HEMIPLICATA, HALL. 
133 
another plication. It is, then, a mature specimen of that class 
which has no plications. In the same way figure 3, Plate IV, 
appears to be a mature specimen with one plication, figure 11, 
Plate IV, an adult of two plications, etc. That is, there are 
specimens which have not attained the maximum number of 
plications in the sinus although they have reached maturity. 
They, then, must have commenced growth with a tendency to 
form no plications, one, two, or three, as the case may be, but 
not the maximum number. 
Second. — The adult specimens illustrated by figures 1, 3, 6, 
9, 11, 20, and 30, Plate IV, appear to have reached their full 
development of plications and most of them have formed their 
maximum number while still far from reaching their present 
mature size, the last stages of growth being in size only. These 
specimens, which are quite typical, have evidently commenced 
growth with the inherent tendency to form a certain number of 
plications, and have ceased forming new ones when the required 
number was reached, although growth is continued. On the 
other hand there are a few specimens, that cannot be ignored, 
such as those illustrated by figures 9 and 24, Plate IV, which, 
though adults, are evidently in a transition stage. It is prob- 
able that each of them has reached its mature form, but the 
next generations might appear with three and five plications, 
respectively, in the sinus. 
It seems, then, that each shell has commenced with an inherent 
tendency to form a given number of plications — whether it be 
one, two, three, or the maximum number — and, with a few 
exceptions, has ceased to form more when the required number 
is reached, though growth is continued. 
Development of One Group from Another. First. — Considering 
the specimens at the stage of growth when the plications are 
initial, it is evident, from the illustrations given, that when a 
shell is of the plication-forming variety it does not usually leap 
at once into its adult condition of two, three, or four plications, 
as the case may be, but after the formation of the sinus the 
other plications appear, sometimes one at a time, or sometimes 
two, in some cases more. The initial stages of the plications show 
two tendencies in each group of shells. The majority of the speci- 
