MOLLUSCA PROPER. 195 
there is only one adductor muscle, but ordinarily there are 
two (fig. 101, 2). These muscles leave distinct scars or “ mus- 
cular impressions” in the dead shell, so that it is easy to 
determine how many were present in any given shell. The 
margin of the mantle, too, is muscular, and leaves upon the 
shell a distinct line where it was attached, this being known as 
the “ pallial line” (Lat. Zallzum, a mantle), as shown in fig. IOI. 
As regards the shell of the bivalves the following are the 
chief points to be noticed. Each valve of the shell’ (fig. 101) is 
to be regarded as essentially a hollow cone, the apex of which 
is turned more or less to one side. The apex of the valve is 
known as the “umbo” or “ beak,” and is turned towards the 
mouth of the animal. Consequently, the side of the shell to-~ 
wards which the beaks are turned is known as the “anterior” 
Fig. toz.—Shells of Lamellibranchiata. 1. Cyclas amunica, a shell with two 
adductor muscles, and an ‘“‘entire” mantle-margin. 2. Tapes pullastra, a 
shell with two adductors, and an indented pallial line. 3. Perna ephippiume, 
a shell with one adductor muscle: @ Pallial line; 4 Scars left by the adductors; 
¢ Siphonal impression. 
side, and it is usually much shorter than the opposite or “ pos- 
terior” side. The side of the shell at which the beaks are 
situated is known as the “ dorsal” margin; and here the valves 
are united to one another for a longer or shorter distance along 
a line which is known as the “hinge-line.” The union between 
the valves is usually effected by means of interlocking parts 
or “teeth,” and there is often a band of horny fibres passing 
between the two valves just behind the beaks. In many cases 
there is also a series of horny fibres placed perpendicularly 
between the beaks, so as to be compressed when the shell is 
shut. By the elasticity of these, and of the external ligament 
when present, the valves of the shell are opened, without any 
effort of the animal, simply by relaxing the adductor muscles. 
