from the Shales of the Northumberland Ooal-field. 361 
a low-pitched roof; and at last it entirely disappears, and is 
replaced, as it were, by a somewhat obtuse point of dentine. 
In the finest specimens, the whole tooth below the enamel- 
wp is coated with a distinct film of enamel, which is perfectly 
colourless; in others traces of it are observed only here and there; 
but in by far the greater number it is entirely wanting: when 
this is the case, the surface of the tooth is frequently observed 
to be roughened, as if by erosion. And it may be here stated 
that it is not merely the enamel that is eroded, but it frequently 
occurs that in the teeth of Palwoniscus, as well as in the teeth 
of other small fishes, the dentine itself is worn away to such 
an extent that very little of it is left to protect the pulp-cavity. 
It is, therefore, not unlikely that all the teeth of Palwoniscus 
were originally coated with enamel; or it may be that in some 
species there is an external coating of sincnisl and in others it 
is wanting. When the tooth is perfect, its walls are thick in 
proportion to the calibre of the pulp-cavity; the calcigerous 
tubes are very fine and numerous. 
Note.—After the above description of the tooth of Paleo- 
niscus, it is scarcely necessary to say that there is no character 
by which it can be distinguished from that of the so-called 
renus Ganacrodus of Professor Owen (pl. 6): the teeth of the 
atter and former agree in size, form, and structure. We have 
found the enamel-tip to exist in P. comtus and other species 
from the marl-slate as well as in the species from our Coal- 
measures. ‘his we have provedin the most satisfactory 
manner, not by taking the teeth at random as they are scat- 
tered through the matrix, but by taking the jaws from the 
heads of well-authenticated Palwonisc’, and examining the 
teeth both externally and in section. After having done this 
in a great number of specimens, we are enabled to state that 
the small enamel-tipped teeth found detached in the Cram- 
lington and Newsham shales are exactly the same as those 
attached to the jaws. They are of the same size and form, 
with the same bright tip of enamel and finely fretted walls ; 
and in section there is no difference whatever; the general 
form, the enamel-cap, the pulp-cavity, and dentine are all 
precisely the same; and all precisely agree with the tooth of 
the so-called Ganacrodus. It is therefore hard to under- 
stand what is meant by the use of such terms as “the 
villiform teeth of Amblypterus and Paleoniscus,” “the vague 
and ill-defined characters of those en brosse of Palewoniscus 
and Amblypterus.” Such expressions may indeed mislead, 
as they or similar words appear to have misled their author ; 
Ann. & Mag. N. Hist. Ser. 4. Voi. i. 26 
