more perfectly burnished than that formed by the outermost, third
layer in the ostrich.

While the eggs of some birds have the shell so thin as to be
translucent, e.g. kingfisher, others display considerable thickness, the
maximum being reached in the egg of the extinct _Aepyornis_.

Though in shape differing but little from that of the familiar hen's
egg, certain well-marked modifications of form are yet to be met with.
Thus the eggs of the plover are pear-shaped, of the sand-grouse more or
less cylindrical, of the owls and titmice spherical and of the grebes
biconical.

In the matter of coloration the eggs of birds present a remarkable
range. The pigments to which this coloration is due have been shown, by
means of their absorption spectra (Sorby, _Proc. Zool. Soc._, 1875), to
be seven in number. The first of these, oorhodeine, is brown-red in
tone, and rarely absent: the second and third, oocyanin, and banded
oocyanin, are of a beautiful blue, and though differing
spectroscopically give rise to the same product when oxidized: the
fourth and fifth are yellow, and rufous ooxanthine, the former combining
with oocyanin gives rise to the wonderful malachite green of the emu's
egg, while the latter occurs only in the eggs of tinamous: the sixth is
lichenoxanthine, a pigment not yet thoroughly known but present in the
shells of all eggs having a peculiar brick-red colour. Still less is
known of the seventh pigment which is, as yet, nameless. It is a
substance giving a banded absorption spectrum, and which, mixed with
other pigments, imparts an abnormally browner tint. The origin of these
pigments is yet uncertain, but it is probable that they are derived from
the haemoglobin or red colouring matter of the blood. This being so,
then the pigments of the egg-shell differ entirely in their nature from
those which colour the yolk or the feathers.

While many eggs are either colourless or of one uniform tint, the
majority have the surface broken up by spots or lines, or a combination
of both, of varying tints: the pigment being deposited as the egg passes
down the lower portion of the oviduct. That the egg during this passage
turns slowly on its long axis is shown by the fact that the spots and
lines have commonly a spiral direction; though some of the markings are
made during periods of rest, as is shown by their sharp outlines,
movement giving a blurred effect. Where the egg is pyriform, the large
end makes way for the sma