THE TEREDO AND ITS DEPREDATIONS. 



403 



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, Cf 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 ro- 

 tation 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 lin- 

 ing 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,/), 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, 



