The Mollusca are so named from the soft nature of their bodies, and are familiar to us as exemplified in the garden-snail and the shells of the sea-shore. The general type upon which they are constructed is that of an external muscular bag, either entire or divided into two, called the mantle, in which the viscera are contained. In most of the orders, they have likewise an outer hard calcareous shell, consisting of one or more parts. It is of course this shell alone that can be detected in the rocks, but by attending to the relations between the living animals and their shells, we ascertain the nature and affinities of the fossil species. Few who ramble by the sea-shore, gathering limpets, whelks, and cockles, are aware how complex an anatomy is concealed within one of those brown discoloured shells. There are elaborate nervous and muscular systems--sometimes several hearts with accompanying arteries and veins--often dozens of rudimentary eyes--capsules which perform the function of ears--jaws, teeth, a strongly armed tongue--gullet, gizzard, stomach, liver, intestine, and complete breathing apparatus. The structure and grouping of these parts vary in the different genera and orders, and upon such variations is founded the classification of the naturalist. Thus, the mollusca of the highest class are called the _Cephalopoda_, or _head-footed_, because their feet, or rather arms, are slung in a belt round the head. They contain, among their number, the cuttle-fish, with its curious internal bone that shadows forth, as it were, the coming of the vertebrate type; and the nautilus, with its many-decked vessel of pearl. The second class is termed the _Gastropoda_, or _belly-footed_, as the genera embraced under it creep on the under side of the body, which is expanded into a broad retractile foot. The common snail and whelk are familiar examples. The third class is formed by the _Pteropoda_, or _wing-footed_--delicate animals, found only in the open sea, and remarkable for a pair of wing-like expansions or fins on the sides of the mouth. The _Lamellibranchiata_ form the fourth class, and receive their name from the laminated form of their branchia, or gills. They contain the two-valved shells, such as the oyster and scallop, and are one of the most abundant groups of animals on our coasts. The fifth class consists of the _Brachiopoda_, or _arm-footed_ molluscs a name given to them from their long spiral arms, once thought to be the instruments of motion, but now ascertained only to assist in bringing the food to the mouth. The sixth, and humblest class, has received the designation of _Tunicata_, from the thick bladder-like tunic, or sac, which supplies the place of an outer shell.