70 
SEA-SHELLS AND THEIR MAKERS. 
with regard to the differences which they exhibit in diffe¬ 
rent genera, for we know much less about the animals of 
Conchifera than about those of the Gasteropoda. 
Bivalve shells should be obtained alive whenever that 
is possible, not only because observations on the animals 
are required, but because when thrown up on the beach 
the valves soon become detached and broken. After notes 
have been made about the points above mentioned, the 
animal can be easily removed by being placed in hot water, 
when the valves open and the muscles which hold the shell 
together can be cut. There are generally two of these 
“ adductor ” muscles, and the shell then opens more widely 
because the valves are pulled apart by an elastic ligament 
above the hinge-plate; while wet, however, they can be 
closed, and should be tied together with thin string or 
cotton. In the case of small shells this is quickly done by 
taking two or three turns of cotton round them and 
twisting the ends together between the fingers without 
troubling to tie a knot. 
Most of the Bivalves live in sand or mud near or below 
low water-mark, but some attach themselves to rocks, and 
a few even bore holes in soft kinds of rock or in timber. 
Some, therefore, can be dug out at low water with a spade, 
and others extracted by breaking off pieces of rock or 
coral-reef at low water; but to obtain those from the sea¬ 
floor outside low water-mark a small dredge must be used 
from a sailing-boat. Occasionally some of these are torn 
from their hold in the sand or mud by storm-waves, and are 
thrown up on sandy beaches without being injured, so that 
such a beach is always worth searching after a storm, or 
after a strong wind from seaward. Most of the Conchifera 
are marine animals, but a few are to be found in the mud 
of estuaries, rivers, and lagoons. 
It may perhaps be as well to mention some of the 
different kinds of Bivalves, for they differ much in shape 
and in texture of shell. Most of them have similar oppo¬ 
site valves, which are either smooth or sculptured, white 
or variously coloured ; such are the ordinary Cockles 
(Gardium), scallops ( Pecten ), and Tellens (Tellina ); others 
are elongate and covered with a dark-coloured epidermis, 
such as the various kinds of Mussels, Mytilus, Modiola , 
Pinna , and Perna ; these are generally attached to rocks or 
timbers by a bundle of fibres secreted by the foot and 
known as a “byssus.” Others, again, have a flatfish 
shell with a scaly or platy structure, such as the Oysters, 
