SHARKS 3 



become enlarged or joined up with their neighbours to form 

 the teeth. 



Another feature of interest concerning the teeth ol Selachians 

 is the fact that instead of being more or less firmly attached 

 to the jaws they are simply embedded in the gums. Further, 

 a Shark does not retain the same set of teeth throughout the 

 greater part of its life, nor is it content with two sets like 

 ourselves, but provision is made for a constant supply of new 

 teeth to replace those in use — a most enviable arrangement ! 

 If the jaws of a large Shark be examined, the young teeth can 

 be seen lying in a shallow cavity on the inner surface of the 

 jaw closed by membrane (Fig. 59). They lie closely packed, 

 one on top of another, with their points directed downwards in 

 the lower jaw, upwards in the upper jaw. Those near the 

 edges of the jaws can be seen to be in a more advanced stage 

 of development and in a more erect position than those further 

 back, and are, in fact, all ready to take the place of the row 

 at present in use as soon as these shall be worn out and cast 

 off. The forward movement of the whole phalanx of teeth 

 goes on throughout life, a row or two doing duty for a time, 

 only to be replaced by their successors, standing by, as it 

 were, in the rear ! 



The number of different kinds of Selachians living to-day is 

 far less than that of the Bony Fishes, but the class includes, 

 nevertheless, a large and diverse assortment of types. The 

 origin of the class may well be said to be wrapped in the mists 

 of antiquity, for our knowledge of the earliest Sharks is based 

 upon the discovery of isolated denticles, teeth, fin-spines and 

 so on, buried as fossils in the rocks of the Silurian and Devonian 

 periods of the earth's history. Since there is reason to believe 

 that these rocks were laid down at least 200,000,000 years ago, 

 the class is a very ancient one, and the very diversity of the 

 fossils suggest that even at this remote age there already 

 existed a wealth of different kinds of Sharks. 



Unlike the Bony Fishes, comparatively few Selachians can 

 be described as of very great value to mankind. Some of the 

 smaller Sharks and Rays, it is true, provide wholesome and 

 nutritious food, and in parts of the Orient there is quite a 

 respectable trade in shark-fins for use in making soup The 

 crude skins of certain Sharks and Rays, with the dermal 



