Marvels of 
[Bu C. EB. Swan. 
The doors of shells. The largest is of a stony sub- 
stance, the others are of horn. 
[By C. EB. Swan. 
The snails have the doorways of their shells inverted 
and drag the apex of the shell along the ground. 
[Bu C. BE. Swan. 
The first two of these shells show breathing channels, which 
are useful when the the 
others are notable for the teeth which guard the entrance to 
the shell 
door of the shell is closed down; 
the Universe 
Umbrella Shell, heavy and very flat ; then come 
instances of more elaborate structures (the 
Slipper and the Cup-and-Saucer Limpets), where 
an interior platform has been introduced for 
the attachment of the muscles. The habits of 
the Slipper Limpets are curious. They live 
piled up one on the top of the other, in the 
same way as bowler hats may be seen stacked 
up by a hatter. 
With regard to the coiling of the spiral, 
some of its many variations are shown on 
page 833. First is the Hungarian Bonnet, 
which is almost limpet-shape, save that its apex 
has begun to coil backwards. 
are formed round a central column or axis of 
the shell, which runs through the whole length, 
just as a winding staircase is constructed round 
an iron shaft. Typical examples of the spiral 
are the Top Shells, where the coil narrows some- 
what suddenly to a point, or the Augur Shells, 
where the narrowing takes place very slowly, and 
so produces a slender and sharply-pointed shell. 
These are the usual forms. Now let us turn 
back to our illustration on page 833. Here we 
have a Worm Shell (7), for all the world like a 
coil of tubing pulled out to its fullest extent. 
This shape is possibly an adaptation of the snail 
to life amongst seaweeds and corallines, where 
the creature could coil itself into the stem and 
branches of the weed and remain securely 
It should be noticed that the central 
column has been dispensed with; so has it in 
the Staircase Shell (6), only here the coils are 
more compact and built neatly one on the top of 
the other, forming a graduated spire. To 
demonstrate this, the shell has been photo- 
graphed from below. Ifa spiral shell be held 
in the hand with the opening uppermost, this 
opening will generally turn to the right of the 
spire. But here are shown two shells (3) of the 
same species of mollusc, each of which coils a 
different way; such cases as this are not un- 
common, but are restricted to certain species, 
while certain other species persistently coil 
towards the left. 
It is no uncommon thing to find a shell in 
which the last whorl of the spire is developed 
out of all proportion to the remainder. Of such 
a type is the beautiful Harp Shell (4), and this 
Now most spirals 
fixed. 
