254 Animal Life 
skin of rays and _ sharks, 
which give to “shagreen” 
its peculiar rough surface 
so admirably adapted to the 
requirements of the polisher. 
There is, in fact, an imper- 
ceptible transition from such 
skin-granules to true internal 
teeth, and we thus learn that 
teeth, like the linmg mem- 
brane of the mouth, were 
originally simple external 
structures, which, with their 
transference to the interior, 
gradually developed the extra- 
ordinary variety they now 
present. 
I have said that in most 
fishes the teeth of the jaws, 
as they become worn out, 
are succeeded and replaced 
ey \ ‘ by new ones. In many cases, 
i 3 = +, Je ea . 
Fig. 2. Jaw of the Port Jackson Shark. } ‘ Be however, especially among 
fi \\\ the sharks and rays, the 
eos / mode of replacement 1s quite 
different from the one obtaining among crocodiles, where each tooth is replaced by a 
successor growing beneath its root (see previous article of this series). In the sharks 
and rays, as well as in many of the enamel-scaled fishes, the teeth are developed on 
the surface of a more or less complete half-cylinder. In the front of the jaws of 
sharks (fig. 2) and throughout those of some rays (Fig. 10) the teeth are arranged in 
straight rows on this half-cylinder, but on the sides of a shark’s jaw the rows are 
oblique, so that a spiral arrangement occurs. As a consequence of this spiral 
arrangement, it results that the lowest and youngest of the teeth are actually immediately 
below the topmost tooth in the same row which they are ultimately destined to 
replace, the crowns of the two teeth at the top and bottom of the series being, of 
course, turned in the opposite directions. This remarkable arrangement is well displayed 
in the largest row of crushing-teeth on the sides of the jaw shown in Fig. 2. As 
the teeth in use on the margins of the jaws become worn and shed, the whole 
half-cylinder gradually revolves on the jaw so as to bring a fresh series of teeth into use, 
while new teeth are developed 
on the basal line, which in life 
is, of course, deeply embedded in 
the gum. Another feature of the 
teeth of sharks and rays is that 
they are always confined to the 
cartilages commonly called jaws, 
although, as a matter of fact, such 
cartilages do not correspond with 
the jaws of other fishes. 
In the most prunitive extinct —_ of Primitive 
ae ae a G 7 E soy Shark Fig. 4. Tooth of the Giant Fig. 5. Tooth of Comb- 
sharks (Fig. 3) the teeth consist (Diplodus). White Shark, much reduced. Toothed Shark. 
