THE STUDY OF SHELLS. 145 



appropriately this fold is known as a Mantle. It lines the 

 inside of the shell, adhering to it some little distance back 

 from the edge. Examine the inside of the shell and dis- 

 cover the mantle line (Fig. 48). 



Two questions may be suggested here. How has the 

 shell come to be as large as we see it now ? It is a hard, 

 insensitive, dead thing, yet it must have grown. It was 

 not always as large as we now see it. How has it grown ? 

 Shells without living animals within them do not grow. 

 And when we remember that the mantle is the part of the 

 animal which lies against the shell, it is not difficult to 

 realise that the mantle produces or makes the shell, 

 adding to it along the outer edge. 



Look now at the outside of the shell. Observe the 

 succession of ridges which pass round it, marking succes- 

 sive additions to the shell. These may be termed lines of 

 growth. By tracing them backward we arrive at the oldest 

 part of the shell. Let the pupils find this part. It will 

 be seen to be turned downward, and nearer to one end of 

 the shell than the other. This part is termed the beak, 

 and it is turned towards the front end of the animal. 

 This is a quite general rule, so that by noting this fact we 

 can tell from the shell where the head end of the animal lay. 



Here is a simple exercise. Let each pupil hold a half-shell. 

 How can we tell whether it is a right half, or a left half ? First, 

 let us name the regions we know. We have just determined the 

 anterior or front end. The opposite end is of course posterior. 

 The free open edge (growing edge) of shell is the under side (ventral 

 side), hence the part where the valves join (hinge region) lies next the 

 back (dorsal side). Now let pupils apply the following rule. Hold 

 the half-shell in the hand with the front end directed upward and 

 with the inside of the shell towards your own body. Next move the 

 shell towards that side of your body which will cause the growing 

 edge to be directed forward, and the hinge margin towards your 

 back. You will occupy the same relative position to the shell 

 which the animal did when living, and if the shell is a right half 

 it will be held at your right side, and if a left half, at your left. 

 Repeat the exercise with several different shells. 



Note next the layers of the shell. On the outside we 

 can recognise a dark coloured horny-like covering. Differ- 

 ent kinds of shells examined will show that it is strong in 

 K.S. 10 



