mollusca: 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 tooth- 

 lets, of which the central one is the largest ; and its base 

 is united with the cartilaginous or gristly substance of the 

 band. Only the middle plate is symmetrical ; the lateral 

 ones bend inwards towards the central one, and are sym- 

 metrical 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 com- 

 posed. The tongue before us has 600 rows such as these, 

 each, as we see, closely following, and indeed overlapping, 

 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 



