146 THE BOOK OF NATURE STUDY 



subject of a school study which will probably remain as a seed 

 in the mind when much, apparently more important, has sunk 

 into oblivion. For besides the large idea that such a minute 

 item as a scale on a fish's back has its distinctive individuality, 

 like the fish as a whole, there is the profoundly interesting fact 

 that the details of the scale form in some measure a record of 

 the fish's life. The concentric rings on the surface of the scales 

 of many bony fishes tell us the age of fish, as do the rings of wood 

 in the stem of a tree. The close-set lines of growth correspond 

 to winter or to wintry conditions ; the wider lines of growth 

 indicate a summer or a period of summer-like conditions. Thus 

 a single scale might not only tell us of the presence of a certain 

 kind of fish, and of no other; it might even tell us the age of 

 the animal! 



(i) In sharks, dog-fishes, skates, and other gristly fishes, 

 the (" placoid ") scales are complex structures. They are 

 tipped with enamel, cored with dentine or ivory, and based 

 with a small plate of bone. There are thus three kinds of hard 

 tissue, of which enamel is the hardest and bone the least hard. 

 The whole scale arises as a papilla of the skin ; the enamel is 

 made by the epidermis, the core and base by the dermis. If 

 a scale of this type be carefully removed it often shows a small 

 hole in the middle of the bony base ; this is where the blood- 

 vessels from the dermis enter the growing scale and feed the 

 pulp in the middle of the ivory. These scales are often called 

 skin-teeth or dermal denticles, and it is noteworthy that the 

 teeth in the jaws of the gristly fishes have a similar structure 

 and a similar development. In the well-known saw of the saw- 

 fish Pristis, etc. there are huge tooth-like scales in sockets on 

 each side of the powerful weapon, which is an extraordinary 

 prolongation of the front of the head. In skates it will be seen 

 that large scales often occupy strategic positions of defensive 

 or offensive importance. Thus they protect the eye and they 

 make the tail an ugly weapon. The skin of shark or dogfish, 

 covered with minute close-set scales, is used for polishing wood, 

 for covering boxes, and the like. It is called " shagreen/' One 

 should demonstrate the fact that when stroked in one direction 

 it is relatively soft and smooth, whereas it feels like the roughest 



