MOLLUSC A: THEIR TONGUES 68 



lateral ones bend inward toward the central one, and are 

 symmetrical only when considered in pairs, each associated 

 with its opposite. The plates are perfectly transparent, 

 but of a yellow horny color; they are very hard, and as 

 they are not dissolved by acids, it has been supposed that 

 their substance is siliceous (having the nature of flint); but 

 they are more probably chitinous, or formed of the sub- 

 stance of which the hard parts of insects are composed. 

 The tongue before us has 600 rows such as these, each, as 



OF TBOCHUS. 



we see, closely following, and indeed overlapping, its pre- 

 decessor; so that we can never look at a single row with- 

 out at the same time seeing others which it overlaps, or 

 by which it is overlapped. 



The specimen which I will now show you is broader, 

 but shorter. It is the tongue of Trochus ziziphinua, a large 

 and handsome shell of regularly conical form, not uncom- 

 mon on our rocky shores. It is perhaps a more interest- 

 ing study than that of the Periwinkle. There are here, 

 you observe, three constituent elements in the pattern. 



