Mollusca. 63 



and the internal features ; it may be found convenient to label part 

 of the features in one valve and part in the other. 



Structure of the Clam Shell. For this work get the thickest and 

 heaviest shells, at least one valve for each member of the class. 

 Weigh them and then roast them by laying them on an old shovel 

 or layer of sheet iron, and placing them on the coals in a stove or 

 furnace. After roasting handle them carefully so as to keep them 

 entire. Weigh them again after roasting and compare with the 

 former weight. 



Hold a roasted valve by the dorsal margin in the left hand, 

 with the inside of the valve toward you. With the fingers of the 

 right hand supporting the outside of the valve, press with the 

 thumb on the ventral border of the valve, outside of the mantle 

 line. The shell should separate into two parts, the inner begin- 

 ning very thin at the mantle line and becoming thicker toward the 

 umbo ; the outer portion extending the whole width of the valve, 

 but becoming thicker from the umbo to the mantle line and thin- 

 ner again from this line to the ventral margin. It will be seen 

 that the plane of division is the plane along which the mantle line 

 has traveled during the growth of the shell. 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. 



Composition of the Clam Shell. Put pieces of the burnt shell 

 into dilute hydrochloric acid. The acid decomposes the limy 

 compound, setting free carbon dioxid. If a fresh shell is placed 

 in acid, the mineral matter will be slowly dissolved, leaving the 

 flexible animal matter, which is called conchiolin. The hinge liga- 

 ment is nearly pure conchiolon, being simply a part of the shell 

 in which no limy matter has been deposited. When the shell is 

 burned the animal matter is burned. The remainder after roast- 

 ing is of about the same composition as lime. Put into acid 

 a piece of the ligament. Does it contain lime? Place an entire 

 valve of a thin-shelled clam in acid for forty-eight hours ; what 

 remains? Are shells ever found in rocks? 



