DAVIS : FOSSIL FISH EEMAINS. 
55 
of the tooth is more or less straight and rectangular, correspond- 
ing with this arrangement of the crown, the under surface, which 
was attached to the jaw, is seen to be concave, the concavity 
forming a channel or grove, widest at the rounded posterior 
extremity of the tooth, and diminishing in diameter as it approaches 
the straight edge. Median teeth may be selected which exhibit 
a similiar decrease from the back towards the front of the tooth in 
the channelled inferior surface whilst both margins are more or 
less straight. The widest end is equal in diameter to the narrowest 
of the previous tooth, and it diminishes forwards and is connected 
with a third and still smaller tooth which occupies the space be- 
tween the median and the s^^mpysis of the two rami composing the 
jaw. The inferior channel on the third tooth diminishes still more 
in diameter, and near the symphysis is almost attenuated to a 
point. In each tooth there is the characteristic convolution. 
One or two specimens have been discovered with long, narrow 
teeth attached in front of the teeth on each side the symphysis 
more or less resembhng those called Helodus ; it appears probable 
that these were attached in front of the anterior pair of teeth, 
which are small and narrow, in order to seize and secure their 
prey with greater facility ; it is also possible that there may have 
been other smaller teeth distributed over the median portion of 
the palate. 
The teeth of the other genera, Deltodus, Deltoptychius and 
Poecilodus appear to have been arranged with modifications to suit 
their several peculiarities in form, in approximately close relation- 
ship with those already described. For the most part they had 
three teeth increasing in size backwards, as indicated by the 
channelled surface attached to the cartilaginous jaw, but it is 
probable that the teeth of the upper jaw of Deltodus and Deltopty- 
chius existed singly, in this respect approaching the arrangement 
in the jaws of Ceratodus. The teeth in both jaws of Poecilodus 
may have been single. 
The Cochliodonts have been regarded by Prof. Owen and 
