PLICATIONS OF PARASTROPBIA HEMIPLICATA, HALL. 
135 
and one only appears to have given rise to the four at the same 
time. Three of the other seven show an original form having 
one in the centre, which bifurcates, and the final two are added 
by implantation. Two more appear to have originated in a 
two-plication form, each of the two original plications being 
bifurcated. The sixth and seventh show that one plication at 
least has its origin in the bifurcation of one of the others, but 
the shells are too worn to show the initial form. One of the 
five-plication specimens has formed its plications simultaneously. 
Two of them form all five by implantation at irregular intervals. 
Two show a derivation from the three-plication form, the first 
by implantation of the subsequent plications, the second by a 
double bifurcation. A fifth shows an initial form of two plica- 
tions, one of which is bifurcated, and later the implantation 
of two. A sixth shows no indication of bifurcation, but there 
are two strong primary plications. A seventh shows that the 
final plication is formed by bifurcation, but the initial form 
is worn. 
Of the two-plication Anticosti specimens of Parastrophia 
reversa (Billings), four form the plications at once, one only 
shows a derivation from the one-plication by bifurcation. 
Three of the three-plication form have arisen from the two- 
plication form, two by bifurcation, one by the implantation 
of the third fold. The four-plication specimen shows one 
bifurcation. 
That, is, then, that the one-plication form appears in the 
initial stages of a number of the two-, three-, and four-plication 
forms, but as far as the specimens examined show dies out in 
the five-plication form. The two-plication form, which seems 
the most firmly established, appears in the three-, four-, and 
five-plication form. The three-plication form is repeated in 
the initial stages of some of the four and five-plication forms. 
The four-plication form appears in one specimen of the five. 
It is worth while also to note here that the two- and three- 
plication shells, the most persistent forms, have a larger pro- 
portion of shells which form their plications simultaneously. 
Second. — Also, though there are exceptions, the majority 
of shells show plications first near the middle of the sinus, where 
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