870 : The American Naturalist. [October, 
of Nautilian shells of the earlier periods are also not so closely 
coiled, or, in other words, remain open and similar to cyrto- 
ceras for a longer time during their growth. This is shown 
by the large size of the central hole, or umbilical perforation, 
left in the center of full grown shells. This perforation is 
much larger, as a rule, in Paleozoic than in the Mesozoic 
forms. 
In each period the genetic series or groups of nautilian 
forms have peculiarities of structure in the sutures, ornaments 
apertures, etc., by which they can be separated from each 
other and these peculiarities are the same as those possessed 
by gyroceran, cyrtoceran and often orthoceran shells which 
occurred often earlier in time so that one can trace each group 
of nautilian shells back to its ancestors through the parallel 
stages of evolution above described. The groups, in other 
words, are parallel in their morphogenesis, like two indi- 
viduals of the same parents in their development from youth 
to old age. 
In all of these cases the impressed zone originates as 
described above after the whorls come in contact, never before 
this time in the growth of any individuals. Barrandioceras 
is one of the most involute shells known in the Silurian, and 
pl. XVIII, fig. 6, gives a true sketch of this species; fig. 7, Shows 
a section of a full grown shell with a decided impressed zone; 
and fig. 8, is the young. This last is a purely cyrtoceran form 
with a compressed elliptical section like that of fig. 7, but no 
impressed zone, the inner side being rounded like the diagram 
of Cyrtoceras, fig. 2. The impressed zone is not present in 
the young of Ophidioceras, the closest coiled of all these forms, 
nor in the young of any species of the Silurian before the whorls 
touch, so far as known, and all of the species likely to present 
this peculiarity have been investigated. 
The impressed zone is also invariably lost in the oldest 
stage of the whorl of every individual when the whorls cease 
to continue to grow in contact. This condition is represented 
in the last part of the outermost whorl of figs. 4 and 5 in sec- 
tions, figs. 4e, 5e, and in the outlines of their apertures which 
are elliptical. The sections represent cuts through the whorls 
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