from the Shales of the Northumberland Coal-field. 349 
Low-Main Seam and Shales of Coal, Northumberland’*. 
It has become necessary to do this, as the anticipated bene- 
ficial results from the former ‘ Criticism”’ of the “ Abstract ”’ 
of the paper as read have not been realizedy, though the in- 
fluence of this criticism is distinctly traceable in the text of the 
published paper, as well as in the appended footnotes. 
The first genus we have to refer to is that named Miogano- 
dus (pl. 8), which is founded on the section of a tooth that in 
no respect differs from that of the so-called Rhizodus lancei- 
jormis, Newberry. We have shown in the former part of this 
communication that this reputed fish is most probably a Laby- 
rinthodont amphibian; but be this as it may, we have teeth 
of this species attached to the dentary bone exactly similar in 
contour to, and not larger than, the figure of the tooth of this 
so-called new genus: and when a longitudinal section of 
these teeth is examined under the microscope, there is no per- 
ceptible difference in the minute structure from that of the 
tooth of Mioganodus; even the concentric layers of dentine, 
which are considered characteristic, are equally well marked. 
Certainly, when the tooth of 2. lanceiformis is perfect, the base 
exhibits the Labyrinthodont infolding of the peripheral wall 
of dentine; but when the tooth is found detached (and that 
figured by Prof. Owen was so found), the basal portion is 
rarely if ever present ; and then the dentinal walls are observed 
to thin out from the interior and to terminate below, when 
_ seen in section, in sharp wedge-shaped points, just as they are 
represented in the figure of Mioganodus ianiartus. The tooth, 
then, on which this genus is founded is merely the upper por- 
tion or crown of a tooth of the so-called Rhizodus lancetformis. 
Rhizodopsis sauroides, sp., Williamson. 
Several specimens of the elegant fish upon which Professor 
Huxley founds the genus Rhizodopsist have occurred at 
Newsham. They are all in a very incomplete state, though, 
with the aid of the whole series, many of the characters can 
be determined. The most perfect specimens are between five 
and six inches in length; the largest is eight inches long, exclu- 
sive of the tail, which is wanting; and the smallest is not 
more than two or three inches in extent. There is proof, 
however, that this species sometimes attains a considerable 
size: a crushed head has been found that measures nearly 
three and a half inches in length; and ossified vertebral rings 
_have occurred that are nine-tenths of an inch in diameter. 
* Trans. Odontological Society, 1867. 
+ Geological Magazine, vol. iv. pp. 323 & 378. 
t Quart. Journ. Geol. Soc. vol. xxii. p. 596 (1866). 
