318 CARTILAGINEI. — RAIAD^. 



We select the latter species for illustration. 

 It is abundantly common all round the British 

 coasts, as Avell as on those of western and southern 



THORNBACK. 



Europe. It attains a diameter of two feet; is 

 brown above with paler spots, and white beneath. 

 The whole upper surface is rough with minute 

 points, and is studded with an irregular number 

 of large hooked spines. The structure of these 

 is peculiar ; each consists of an oval, rather thick 

 disk of white bone, the centre of which rises 

 into a sharp conical point curved backwards, the 

 whole possessing a slight resemblance to one of 

 the prickles on the stem of a rose-bush. A 

 group of small ones of similar form surround 

 each eye, and a row runs down the middle of 

 the back in close series. The others are much 

 larger, and irregularly scattered both over the 

 body and the tail. 



The Thornback is much eaten by the poorer 



