parkinsoni (Plate IX, Fig. 18), while in others it is nearly
transparent, and hardly perceptible. To enable the true colours of a
shell to be seen the epidermis must be removed.

The supposed original form of a shell was that of a volute univalve,
such as the Triton (now Lotorium), or Struthiolaria. To properly enclose
the animal, and make it safe from enemies, an operculum, or lid, was so
formed that when the animal retired into the shell this filled up the
opening. The operculum is usually like a piece of thin, rough brown
horn, and where no reference is made to an operculum in this work, it
must be understood that the operculum is horny. Some shells, such as the
Astralium sulcatum (Plate VI., Fig. 18), and the Turbo helicinus (Plate
VI., Fig. 17), have a shelly operculum; that of the latter being the
well-known cat's eye.

In some shells the operculum is small, in others large, and progressing
step by step we find some, such as the scallop and oyster, with one
side round, and the other (really an operculum) flat and as large as the
shell; until we come to the perfect type with each valve the same shape
and size. Then the operculum disappears, as in the limpet, and the
covering shell becomes smaller and smaller, till in the Scutum ambiguum
(Plate IX., Fig. 23) the shell bears about the same proportion to the
animal that the little bonnet, fashionable a few years ago, bore to the
lady that wore it. The shell is built up of very thin layers of nacre,
or mother of pearl, and calcareous or chalky matter, the thinner being
the layers of nacre the more lustrous and iridescent is the shell.

As would be expected from its isolated position, many of the genera of
New Zealand shells are not found elsewhere. The late Professor Hutton
mentions nine genera in this position.

The dispersal of shells is an interesting natural phenomenon. The eggs
of molluscs are so small that they can easily be carried by currents,
attached to floating seaweed or floating timber, on the hulls of ships,
or in the feathers or feet of our migratory birds, such as the godwit,
which every year travels from New Zealand to Siberia and back. A great
many of our shells are found on the Australian coasts; and a surprising
number are common to both New Zealand and Queensland.

In describing the illustrations, length means extreme length, and by
measuring the shell on the plate the proportionate width can be
ascertained. The illustrations are, generally spe