i62 MOSTLY MAMMALS 



for its perfection as a crushing machine as it is for its 

 intrinsic beauty. 



Even more elegant from an aesthetic point of view are 

 the " millstones " of the Port Jackson shark {Cestracion) 

 and its allies. In place of forming a continuous plate 

 across the palate after the fashion of the eagle-rays, the 

 individual teeth in this group are arranged in oblique 

 bands round the edges and inner sides of the jaws,* showing 

 in the hinder region a melon-shaped swelling of remarkable 

 gracefulness, which would form an attractive ornament for 

 the capital of a pillar. In this melon-like portion of the 

 millstone the individual teeth form bluntly convex oblongs ; 

 those of one row being remarkably larger than all the rest, 

 while the rows in front and behind this do not correspond 

 with one another in size. Examined with a lens, each of 

 these blunt teeth is seen to have a minutely pitted structure, 

 while its median longitudinal line is marked by a narrow 

 smooth streak. New teeth are being continuously produced 

 on the margin of the series on the inner side of the jaw ; 

 and as the outer ones become worn away, the whole series 

 is pushed over towards the edge of the jaw. Proceeding 

 from the larger rows of teeth towards the front of the jaw, 

 it will be seen that as the individual teeth become gradually 

 shorter their smooth median line gains prominence, till it 

 finally develops into the sharply pointed cusp surmounting 

 each of the front teeth. 



As already said, the Port Jackson shark and a few other 

 nearly related species (all of which, by the way, feed on 

 shell-fish and crustaceans) are the only sharks with mill- 

 stones met with in our present seas. And it is fortunate 

 that these have lived on, as otherwise we should never 



* Strictly speaking, the tooth-bearing cartilages of sharks are not 

 true jaws. 



