III. — Sea-Shells and their Makers. By A. J. 
Jukes-Browne, F.R.S., F.G-.S. 
A shell is the hard calcareous covering of a soft-bodied 
animal, and the term should properly be restricted to the 
coverings of those soft-bodied animals which are called the 
Mollusca. It should not be applied to the covering of a 
Grab or a Shrimp, such envelopes being called carapaces. 
The Mollusca include several different classes of animals, 
and they construct a great variety of shells, some forming 
simple conical caps like Limpets, others building spiral 
shells such as the Whelks and Periwinkles, and others, again, 
protecting their sides by two separate shells or valves which 
are hinged together at the top as in Cockles and Mussels. 
The study of shells is known as Conchology, and when 
combined with that of the animals which make them it is 
often called Malacology. Every shell is, of course, closely 
related to the animal which made it, reflecting as it were 
the structure of that animal; but there are parts of the 
animal which have little to do with the construction or modi¬ 
fication of the shell. These parts may vary in different 
kinds of molluscs, and consequently it is necessary to 
observe them while the animal is alive, in order to obtain 
a full knowledge of the creature and its relationships to 
others. 
Shell-bearing Mollusca are divisible into three great 
classes : the Cephalopoda or Cuttle-fish ; the Gasteropoda 
or Snails, whether land, freshwater, or marine ; and, lastly, 
the Conchifera or Bivalves, such as Oysters, Mussels, and 
Clams, most of which live in the sea, though some inhabit 
rivers and lakes. 
I do not propose to say much about the Cephalopoda 
because very few of them construct an external shell. 
Many of them possess an internal one, which is sometimes 
horny and sometimes calcareous in composition, the one kind 
being known as “ sea-pens,” the other as “ cuttle-bones, 5 ’ 
and these are often thrown up on the shore. The shell of 
the Argonauta, however, is an external and very beautiful 
construction, consisting of a delicate white convolute shell, 
which is only produced by the female and serves as a pro¬ 
tection for the eggs, which are fixed inside the spire. The 
