134 
MUSEUM BULLETIN NO. 2. 
mens exhibit plications formed at intervals, sometimes by implan- 
tation of a later one, sometimes by the bifurcation of a plication 
which has already grown to some extent, the latter method 
of increase in number is the more frequent. In each group, 
however, there are some shells which form their plications 
simultaneously, never adding to them, though growth is continued. 
It seems probable that the more variable specimens give rise to 
the forming of new groups with more plications, while those 
which form their plications simultaneously have reached a 
constant form in that group. Figure 23, Plate IV, for instance, 
forms its three plications simultaneously, exhibiting no tendency 
to variation. It has become constant. Figure 14, Plate IV, 
on the other hand, shows a primary plication with two added 
later. Some of the descendants of this shell would probably 
tend to a constant form like figure 23, Plate IV, showing the 
final number simultaneously, others might bifurcate the primary 
plication and with the two added form a four-plication shell. 
Owing to the worn condition of many of the shells it is not 
always possible to tell th§ origin of the plications, but, of thirty- 
four two-plication specimens examined, twenty-one appear 
to have formed the two-plications simultaneously, about three- 
fourths of the remaining thirteen either show faint traces of 
an original one-plication form with the one plication bifurcated 
or else they converge so as to suggest that such has been their 
origin. A very small minority seems to form the second pli- 
cation by implantation after a period of growth of the first. 
Of forty-three three-plication shells studied, eleven only appear 
to have simultaneously formed plications, while thirty-two 
have produced plications at irregular intervals. Seven of 
these show a strong one-plication form first, with the subsequent 
plications introduced by implantation, one at each side, usually 
at different stages of growth. Four show an original strong 
one-plication at one side of the middle and the subsequent pli- 
cations, unequal in size, introduced by implantation at one side. 
Twenty-one give definite evidence of having been derived 
from the two-plication form, eight by the implantation of a 
third fold and thirteen by the bifurcation of one of the two 
original plications. Eight four-plication specimens were examined 
