42 Lessons in Zoology. 



held together. The shells thns pTepared can be pnt into the hands 

 of yoang children, who can afterwards see the principal soft parts, 

 snch as the mantle, the gills, and the foot, from the teacher's 

 specimen. 



The clam shell consists of two valves, which are con- 

 vex on one long edge and nearly straight on the other, 

 and which meet at a " rounded point," called the heak. 

 It is hroad at one end and narrower at the other, hut 

 both valves are alike in size and shape. To find the 

 right and left valves we hold the shell with the beak up- 

 permost and the narrow end pointing toward us, when 

 the right valve will be on our own right side, and the 

 left valve on our left. 



The surface of the valves is not smooth, but roughened 

 by many curved lines. Beginning at the beak and 

 tracing all the lines between this point and the convex 

 edge of the shell, we find that they all start at the upper 

 side of a valve and pass around to the upper side again. 

 Children will quickly see that the baby clam could have 

 had but few of these lines, and that more have been 

 added as it grew, hence these are lines of growth. Now 

 they will be interested in tracing these back from the 

 outer edge to the beak, and seeing that this is the shell 

 of the baby clam and the oldest part of the whole shell. 

 A brown, horny skin covers the lower part of the valves, 

 but is worn away near the beak. 



A few of the thickest shells have been roasted on a bed of glow- 

 ing ooals, and pieces of them are now distributed to the class. 



The roasted shells are white and crumble easily. The 

 lines of growth are the edges of layers of lime, which 

 can now be peeled oS. The shells are not so heavy as 

 they were before. 



Since the shells weigh less after the roasting, it is evident that 

 some part of them has been bnrned awa;. To discover what was 



