DAVIS: FOSSIL FISH EEMAINS. 
59 
character obtains in all the Squali or sharks properly so called. 
In other g-roups with a flat and crushing- dental arrangement, the 
old teeth are replaced by new ones which grow underneath them, 
or like the CochUodonts, the old portion becomes inrolled on the 
jaw and a new surface is produced by additions to its lateral edges 
as already explained. The Petalodonts were very differently at- 
tached to the jaw ; a strong and deeply-imbedded attachment to 
the jaw g-ave them a solidity more analogous to some of the 
Teleosteans, as for example, Sargus, or the higher vertebrates, 
than to the sharks, and in this respect the genus Polyrhizodus is 
more distinguished than its fellows by having- the roots divided, 
and by that means being- still more firmly attached to the jaw. 
The g-enera Ctenopetalus, Ag., and Petalopsodus, Davis, are 
distinguished, the first by its serrated or crenated cutting surface, 
and the latter by having the coronal edge divided into three dis- 
tinct and almost equal parts, each risings to a finely pointed apex. 
The root is not divided. The remaining representative of the 
group, the Petalorhynchus, Agassiz, is much the most abundant 
and has been found in large numbers. It is also very fortunate 
that several specimens have been discovered which throw consid- 
erable light on the general dentition of the genus. It is now 
known that the teeth were arranged in a semicircle conforming to 
the shape of the jaw in which their long roots were deeply implant- 
ed. The root in the older teeth is frequently four times the depth 
of the height of the crown. The crown has the usual Petalodont 
character, except that its central portion is produced and pointed, 
and is slightly bent inwards so as to resemble the pointed beak of 
a parrot. One central tooth was implanted on the symphysis of the 
two rami of the jaw, and on each side there extended three 'others, 
making a total of seven to each jaw. Specimens have also been 
found shewing how the teeth are replaced by successive larger 
growths, as the mouths of the fishes has increased in size. From 
these it appears that originally there was a single row of small 
teeth extending along the surface of each jaw, that, as these 
