MOLLUSCA I THEIR TONGUES. 47 



structure, and even the same elements, exist throughout. 

 Concentrating our attention on a single transverse series 

 of the numerous curved lines that at first sight bewilder 

 the mind, we perceive, by delicate focussing, that the 

 object before us consists of a number of hooks projecting 

 from the surface of the translucent band, and arching 

 downward. In this case a single row consists of seven 

 such hooked plates or teeth ; one in the centre and three 

 on each side. Each hooked plate has its arching tip cut 

 into five toothlets, of which the central one is the largest ; 

 and its base is united with the cartilaginous or gristly sub- 

 stance of the band. Only the middle plate is symmetrical ; 

 the lateral ones bend inwards towards 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 colour; 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 substance of which the hard parts of insects 

 are composed. The tongue before us has 600 rows such as 

 these, each, as we see, closely following, and indeed over- 

 lapping, its predecessor ; so that we can never look at a 

 single row without 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 ziziphinus, a large 

 and handsome shell of regularly conical form, not un- 

 common on our rocky shores. It is perhaps a more 

 interesting study than that of the Periwinkle. There are 

 here, you observe, three constituent elements in the pat- 

 tern. First, a delicate glassy central tooth, tapering to a 

 fine point, and cut into minute saw-teeth along each edge. 

 Then a series, of five on each side, of similar glassy 

 pointed leaves, bending inward ; and outside these, on 

 either hand, are a great number of stout dark-coloured 



