several raised, wavy ridges showing the form of the very young shell. 

 The shell grows by the addition of new shelly matter, and each time 

 new material is added a distinct line is left on the shell. These we call 

 lines of growth. At intervals, a more distinct line edged with black 

 is formed, which shows where the animal rested and stopped work on its 

 shell. These are called rest periods, and by their aid we are enabled 

 to tell the age of a clam. For example, the shell we are studying is five 

 years old, as there are five of these rest periods, counting the last. 



"If the shell of the clam is held in the hand with the umbones 

 uppermost and the long end nearest the observer, the right and left 

 sides of the clam are in their 

 natural position and the point 

 nearest the observer is the 

 posterior end, the point far- 

 thest away is the anterior end, 

 while the upper, or rounded 

 part, is the dorsal margin, 

 and the sharp, lower part is 

 the ventral margin. The lig- 

 ament and umbones are also 

 in the center of the shell. 



" The shell of the clam, 



ct 



as well as those of nearly all 



Fresh-water clam, showing interior of shell. A, 

 anterior end ; P, posterior end ; a, anterior adductor 

 muscle scar ; b, posterior adductor muscle scar ; c, 

 anterior foot retractor muscle scar ; e, posterior foot 

 retractor muscle scar ; ct, cardinal teeth ; It, lateral 

 teeth; 1, ligament; lu, lunule; pi, pallial line; u, umbo. 



mollusks, is covered by a 



layer of animal matter called the epidermis This protects the more 

 limy part of the shell, which is composed of carbonate of lime with 

 a little animal matter, from being acted upon by the carbonic acid 

 in the water. This epidermis, or skin, is formed by the edge of the 

 soft part of the animal, called the mantle, which lines the shell If a 

 break occurs in the edge of the shell, it is perfectly repaired by the 

 animal and covered by the epidermis ; if, however, the break occurs in 

 any other part of the shell, the damage is repaired by the addition of 

 new shelly matter, but without the epidermis. This shows conclusively 

 which part of the animal secreted this covering. 



"Having studied the outside of the shell," said Professor Parker, 

 " let us examine the inside of this dead clam, from which the animal has 

 been removed by the waves. The inside of the valve is concave, and is 

 marked by several very distinct characters. Near the dorsal margin there 

 are two long, thin teeth, or ridges, called the lateral teeth, and two 

 short, conical tubercles just in front of the umbo, called the cardinal 

 teeth. These interlock with similar processes in the opposite valve. 



