2. The middle part (Fig. 3, b; Fig. 6, b; Fig. 7, b), which is the largest, is strongly arched, and presents, when seen from the side, the form of a half-moon; its ventral portion is a little more pointed, curved inwardly, and terminated by a small swelling or tubercle (Fig. 4, b'; Fig. 7, b'), which, when the shell is closed, comes in contact with the similar tubercle on the opposite valve.
3. The anterior part, which is a continuation of the upper portion of the preceding part, and is more or less spiral in form (Fig. 6, c; Fig. 7, c; Fig. 3, c c; Fig. 4, c c), and its edge makes, when seen from the side (Fig. 6), an angle of a little more than 90° with the free edge of the middle part. The limit of these two parts is marked by a zigzag line, which resembles a sort of suture (Fig. 3, e; Fig. 6, e e). This part of the shell curves backward and inward, and there terminates in a small rounded tubercle (Fig. 4, d; Fig. 3, d; Fig. 7, d), situated opposite the corresponding tubercle of the other valve. This point is the axis of rotation of the two valves; that is to say, when the shell opens or closes, the tubercles retain their relative positions, while all the other portions of the valves describe about them an arc of circle more or less large.
On each of these tubercles is a short, pointed projection, on which are implanted at about a right angle two other large projections, which extend into the interior of the shell a third or half its length (Fig. 4, e e; Fig. 7, e). These projections are slightly curved and flattened; they penetrate among the soft parts, so that their inner face reposes upon the visceral mass; their outer face comes in contact with the thin lining or mantle, which covers the interior of the valves and extends to their extreme edge.
Examining the shell with a magnifying-glass, one sees (Fig. 6) a large number of curved lines of growth, parallel, as is usual, with the free margin of the shell; a closer examination shows that those lines differ in each of the three parts of the valve, although in fact they form a continuous whole.
On the back part of the neck of the valve (Fig. 6, f), the lines seem to be simply curved lines without any especial peculiarities. This is equally true of those on the posterior and largest portion of the middle part of the shell (Fig. 6, b); they seem to be only linear thicknesses; yet, between each pair of the strongest lines, which are the lines of growth, properly so called, one discovers a multitude of others, much liner, which follow the same direction. Here (Fig. 6, e e) the lines of growth form partitions between as many rows of small, sharp,wedge-shaped teeth. Each of these teeth has two rectangular faces on either side of two small triangular faces inclined toward each other (Fig. 8); its cutting edge is placed in the direction of the axis of the animal.
The size of those small teeth varies according to the position they occupy: those which are in the vicinity of the hinge-border are the smallest, those which are on the outer edge are the largest. And, as the part of the shell which is nearest the hinge is the earliest formed,
