60 RADUUE 



from a polariscopic view), down the centre of 

 which runs a narrow stripe, and every band of 

 colour on one side is repeated in the same 

 relative position on the other side. The middle 

 teeth are known as the ' rachidian,' the teeth 

 next adjacent on each side are known as 

 1 laterals/ while the outermost are styled 

 ' uncini ' or ' marginals.' 



In the radula of the common whelk (Buc- 

 cinum undatum) there are about two hundred 

 and fifty teeth. In that of the common peri- 

 winkle about three thousand five hundred. In 

 the Ear Shell (Haliotis tuberculatus) the odonto- 

 phore is well developed. The teeth on the 

 median line are flattened out, recurved and 

 obtuse, those on the inner or first rows of 

 laterals are trapezoidal, while those known as 

 the ' uncini ' are generally hooked. The odon- 

 tophore of this creature seems to be most 

 complicated, the teeth vary in shape so much. 

 In fact, those of the inner laterals may be 

 compared to those of the shark, but are ex- 

 tremely minute. Instead of one row each, the 

 outer laterals contain several rows of teeth. 



