MOLLUSCA : THEIR TONGUES. 53 



cover tluit, -with some modilications of form, the same 

 essential plan of structure, and even the same elements, 

 exist tlirongliout. 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 ribbon, 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 nnited with the cartilaginous snbstanec of the 

 ribbon. Only the middle plate is symmetrical ; the 

 lateral ones bend inwards towards the central one, and 

 are symmetrical only when considered in pairs, each as- 

 sociated with its opposite. The plates are perfectly 

 transparent, but of a yellow horny colour ; they are 

 veiy 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 chi- 

 tinous, 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 fol- 

 lowhig, 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. 



Tlie specimen which I will now show you is broader, 

 but shorter. It is the tongue of Trochus sizip/mius, 

 a large and handsome shell of regularly conical form, 

 not uncommon on our rocky shores. It is perhaps 

 a more intei'esting study than that of the Periwinkle 



