mussel, it is necessary to pass a sharp blade between the valves, and cut through the muscles, before the valves will open. These muscles are called the adductor muscles, and the scars or impressions on the valves are called the adductor muscular impressions. Very close to the adductor muscular impressions are seen smaller impressions, and these indicate where the muscles are attached which move the foot. These muscles are called the pedal muscles, and the impressions are called the pedal muscular impressions. One occurs just behind the anterior adductor impression; the other will be found just above, and in front of the posterior adductor impression.

FIG. 39. THE RIGHT VALVE OF A FRESH-WATER MUSSEL.-C, Cardinal Teeth; 7, Lateral Tooth; li, Ligament; aa, Anterior Adductor Impression; pa, Posterior Adductor Impression; ap, Anterior Pedal Muscular Impression; pp, Posterior Pedal Muscular Impression; p, Pallial Line.

34. Besides these marks, the pupil will see a delicate and slightly irregular line running from the anterior to the

25. In the land snails, it was learned that the eggs were deposited separately, while in the air-breathing, freshwater snails the eggs were inclosed in a gelatinous substance. Among the sea snails there are many species which inclose their eggs in pods, or capsules, as is also the case in the operculated fresh-water snails. Sometimes the capsules are clustered together in large masses, as in those of the whelk (Fig. 31):

FIG. 31.- A VERY SMALL CLUSTER OF EGGS OF THE WHELK DEPOSITED ON THE STEM OF A LARGE SEA-WEED.

In others they are united in a long string (Fig. 32).

FIG. 32.-A, a Portion of a String of Egg-Capsules, from a Florida Species of Whelk similar to Fig. 27; B, a single Capsule separated, showing Outlet, o, through which the Young escape; 8, Stem.

The common cockle sticks its separate egg-capsules side by side upon the rock (Fig. 33).

The beach-cockle deposits its eggs in a broad ribbon of sand cemented together, looking very much like a deep saucer, with the bottom broken out, and the side separated (Fig. 34).

FIG. 33.-EGG-CAPSULES OF COMMON COCKLE.-(A shell of the animal which produces these capsules is shown in the upper central figure of Fig. 26.)

This ribbon is elastic when wet, and, if it is held up to the light, the little transparent spaces for the eggs will be plainly seen.

FIG. 84.-EGG-RIBBON OF BEACH-COCKLE (the shell of which is shown in the lower right-hand figure of Fig. 20).

CHAPTER V.

FRESH-WATER MUSSELS.

26. LOOKING over our fresh-water shells again, we find many that are known as mussels, or clams. These shells are common everywhere along the margins of brooks, rivers, and lakes. The musk-rats feed upon the soft parts of the mus

sels, and the remains of their feasts may be found in piles of mussel-shells, all along the shores of certain lakes.

The shell is composed of two pieces, or valves, as they are called. The two valves are often found united, and the margin along which they are connected is called the hingemargin, because the shells hinge at this part, and will open and shut as a door swings upon its hinges.

Let the pupil now examine a perfect fresh-water mussel, that is, a mussel in which the valves are united in this way, and he will observe that they are connected by a brownish substance, which is quite elastic when the shell is alive, but becomes brittle when dried. The shells are held together as the covers of a book are held together by the back.

This substance is called the ligament, and the position of this ligament will indicate the back, or dorsal region of the animal.

27. On the outside of the shell will be seen fine lines, which run nearly parallel to the outside margin of the shell. These lines are the lines of growth, and indicate the successive stages of growth, or increase of the shell, as in the lines of growth in the snail-shell already studied, and, as in the snails, the growth takes place at the margin of the shells.

The pupil may trace these concentric lines back, as they grow smaller and smaller, till they are found to start from one point at the back of the shell, and this point is called the beak or umbone. It represents the starting-point in the growth of the shell. In fresh-water mussels, the umbones are eaten away by some corrosive action of the water, and the

early stages in the growth of the shell are usually destroyed. In very young shells, however, the early stages can be plainly

FIG. 35.-A FRESH-WATER MUSSEL.-, Ligament; u. Umbone; f, Foot; ex., Excurrent Orifice; in., Incurrent Orifice.

28. The ligament is always behind the beak, or umbone, in fresh-water mussels, and in nearly all bivalve shells (so called, because they have two valves or pieces, while the snail-shells are sometimes called univalve shells, because they have but one valve or piece).

Let the pupil now hold a perfect mussel-shell in his hand (that is, a mussel in which both valves are together, and united across the back), with the ligament uppermost, and the umbone away from him, or beyond the ligament, and the valve on his left hand is the one which covers the left side of the animal, while the valve on his right hand covers the