162 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 (Cestraczon) 
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. 
