CHAPTER VII. . 

 BRANCH MOLLUSCA. 



THE branch Mollusca includes clams, oysters, scallops, 

 snails, slugs, squids, and cuttlefishes. The large majority 

 of mollusks have shells. The shells have always been 

 objects of great interest on account of their beauty of 

 color, delicacy of texture, and variety of form. Their 

 durability and the ease with which a collection may be 

 made and kept have further contributed to making the 

 mollusks a favorite subject of study. 



CLASS PELECYPODA. 

 Example. The Fresh-water Clam. 



Where Clams Live. Clams live in creeks and ponds, 

 lakes and rivers. They are usually less abundant in the 

 smaller streams, as these may become dry in midsummer. 

 The natural position of the clam is shown in Fig. 64. 



External Features of the Clam. The clam shell is com- 

 posed of two equal valves, fastened together at the dorsal 

 margin by a tough, elastic membrane, the hinge ligament. 

 Somewhat nearer the anterior than the posterior end 

 is a raised point close to the dorsal margin ; this is the 

 umbo ; it is frequently more or less worn or broken away, 

 which is not surprising, as it is the oldest part of the shell 

 and is subject to friction as the clam plows along through 

 the sand. Around this are the concentric lines of growth, 

 running parallel to the ventral margin. Different species 



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