Be TEETHING AMONG SELACHIANS. 
stand how teeth of a large size could follow the teeth of very 
small dimensions which exist in the young. In the teeth of the 
young taken from the mother, embryos therefore, and not young 
in the ordinary sense, the whole width of the dentary portion of © 
the jaw is not half the width of the central tooth of an adult, but 
on careful examination it will be seen that the hind part of these 
teeth is not as broad as the front part. They form in fact cones, 
and of course a cone long engugh will bring to the front part very 
small teeth. .Teéth as small as a pin’s head are actually found in 
the jaw. I have examined a number of jaws of Myliobatis, all of 
dropped as remain in the jaw. The teeth are not pushed sid 
wise : they are pushed forward. This occurs in Myliobatis, where 
` we have a number of teeth varying from a single one in the centr 
of the jaw, to four rows on each side and a middle row in addi- 
tion, all of which progress from behind forward. In the g 
ZEtobatis the teeth are conical, the front part being much nar- 
rower than the hind part. 
The introduction of the new kind of teeth is a a complicated thing, 
to explain, and to state the changes which they undergo by! 
would océupy too much time. Let me say only, that beside 
principal modes of teething we have some modifications of 
which are characteristic of particular families. In the 0 
skates (Raja) the rows of teeth are disconnected from one 4 
and run from the margin inward in unbroken continuity. 
| sting-taye (Trygon) and all the representatives of that 
. 
~ 
a that the whole surface of the jaw is covered with teeth. t 
poe r the teeth of the common skate had been brought together 
ri crowded, so that one row was pressed into the space of 
the teeth do not drop singly but are brougbt forward like 
ment, the margin of which is gradually dropping. 
There is another modification characteristic of the 
eanthias) in which the teeth of the successive rows Ov 
ey ated from the side; not uniformly from both sides but 
ai Ar ne overlaps the other tooth. and 
ad bee squeezed from the side and made t 
