62 Practical Zoology. 



THE INSIDE OF THE CLAM SHELL. 



1. Observe the color of the lining layer. Is it uniform? How 

 would you describe the surface finish. 



2. The hinge teeth. These are of two sorts : (i) the cardinal 

 teeth, blunt, toothlike projections near the umbo ; (2) the lateral 

 teeth, long, ridgelike projections below the hinge ligament. Note 

 how many of each of these kinds there are in each valve. How 

 do they fit into each other? What is their use? Do you find 

 them in all clam shells? 



3. The muscle scars: (i) the anterior adductor muscle scar, 

 near the anterior dorsal margin ; (2) the posterior adductor muscle 

 scar, near the posterior dorsal margin; (3) the anterior retractor 

 muGcle scar, just above and back of the anterior adductor scar, not 

 very distinct ; (4) the posterior retractor muscle scar, just above 

 and in front of the posterior adductor scar; (5) the protractor 

 muscle scar, just below and back of the anterior adductor scar ; 

 (6) the mantle line, running parallel to the ventral margin, from 

 the anterior adductor to the posterior adductor scar. 



4. The movements of the muscle scars. When the clam was 

 smaller than it now is, where were the adductor muscle scars? 

 Can you see any traces of the positions of these muscles at an 

 earlier stage of life? Is there an evidence of growth over any 

 part of the muscle scars that now show? Does the mantle line 

 shift its position with growth? 



5. The ligament. Examine the hinge ligament where it was 

 broken off. Has it a definite structure ? What is its chief charac- 

 teristic? 



6. Take an empty shell with the valves still hinged together ; 

 cut and fit into this a piece of paper showing the shape of the 

 whole mantle. 



7. Make a plaster of Paris cast of the inside of a whole shell. 



8. Make a drawing of the inside of the right valve, labeling all 

 the features above noted. 



Take a large flat shell and label (with ink) both the external 



