THE NATURAL HISTORY SURVEY. 37 
which interlock with a single tooth in the right valve. These 
teeth are pyramidal in form. The lateral and cardinal teeth 
form the hinge, which with the ligament serve to guide the two 
shells in opening and closing. Above the cardinal teeth the 
umbo (u) is seen. At either end of the shell there is a scar 
which represents the basis of attachment of the anterior (a) 
and posterior (b) adductor muscles. A second smaller scar is 
seen posterior to the former, and is known as the anterior foot 
retractor muscle scar (c). Above the posterior adductor mus- 
cle scar there is a small scar which represents the attachment 
of the posterior foot retractor muscle (e). These scars arealso 
known technically as cicatrices (singular cicatrix). In the cav- 
ity of the beaks are situated a number of small scars, the dorsal 
muscle scars, which represent the attachments of the small 
Fie. 2. 
Diagram of a section of Unio, showing method of opening and closing the 
valves. I., ligament; II., III., the umbones; IV., V., lateral teeth in the left 
valve; VI., lateral tooth in the right valve; VII., VIII., left and right valves 
of shell; IX., adductor muscle; X., XI., ventral surface of shell, or long arms 
of the lever. (After Lankester.) 
muscles which hold the umbonal and dorsal part of the animal 
to the shell; they are numerous and deeply impressed. 
As before remarked, the mantle or pallium secretes the shell 
substance (carbonate of lime); the edge of the pallium deposits 
new shell matter and adds to the size of the mollusk, It is 
firmly attached to the shell along a line which runs from the 
posterior to the anterior muscle scar, about half an inch from 
the ventral margin. This attachment is called the pallial line 
(pl) or pallial cicatrix. 
The method of opening and closing the shell may best be 
represented by the accompanying diagram (Fig. 2, after Lan- 
kester) which is a transverse section of Unio, cutting through 
the ligament and adductor muscle. The two shells of a pele- 
cypod form a double lever with the hinge teeth (IV., V., VI.) asa 
