PLAGIOSTOMES. 



353 



general form, which differs but little from that of 

 ordinary fishes. Their skin is covered with a mul- 



\MMER-SHAF.K AND SAW-FISH. 



titiide of small spines of stony hardness, and becomes 

 very rough on drying, so as to form a sort of file 

 (shagreen), much employed in the arts for polishing 



Fig. 286.— shark's egg. 



