54 PRACTICAL ZOOLOGY. 



when the shell is shut. The irregularly shaped teeth 

 near the umbones are the anterior, or cardinal teeth; 

 back of these are the long, narrow posterior, or 

 * lateral teeth. 



18. Take an empty shell with the valves still hinged 

 together ; cut and fit into this shell a piece of paper 

 showing the shape of the whole mantle. Make also a 

 plaster of Paris cast of the inside of the shell. 



19. Wipe dry a number of shells and weigh them. After 

 thoroughly roasting them in a fire, weigh them again. 

 If the shells are not burnt too long, some of the 

 animal matter may remain in the form of layers of 

 charcoal between the layers of lime. 



20. Put a shell into dilute acid to dissolve out the lime. 

 Observe the animal matter remaining undissolved. 

 Compare the effect of acid on equal pieces of burnt 

 and unburnt shell. 



21. Carefully pick to pieces a burnt shell and distinguish 

 an inner and an outer portion of the shell, the line of 

 division between which appears on the inner surface 

 of the shell along the mantle line. Make out the 

 successive layers of the outer part which were built 

 by the outer part of the mantle, and the layers formed 

 by the thinner, inner part of the mantle. 



22. Break a burnt shell across from the umbo to the 

 ventral margin, and make a drawing of the edge thus 

 exposed, showing the arrangement of these sets of 

 layers. File a groove from the umbo to the ventral 

 margin of a fresh shell, and break it across. Compare 

 the edges of this with the corresponding part of the 

 burnt shell. 



