from the Shales of the Northumberland Coal-field. 357 
his assertion that “la substance qui forme la surface extérieure 
de la dent est parfaitement homogéne, sans trace de structure 
queleonque.” If his sections had been made very thin, this 
substance would undoubtedly have appeared so. The exami- 
nation of many specimens is frequently necessary to correct 
errors of this nature. 
Note.—It is on the palatal tooth or plate of Ctenodus, 
probably of Ctenodus obliquus (or, perhaps, C. elegans, or it 
may be on a minute plate of one of the larger species) that 
Prof. Owen has founded his genus Saganodus (pl. 12). This 
is one of the genera on which no remark was made in the 
* Criticism ” of the “‘ Abstract ;”’ but a mere glance at the figure 
in the paper is sufficient to satisfy us that 1t represents nothing 
else than a small imperfect palatal plate of this genus. One 
of the authors of the present communication has had in his 
cabinet for many years numerous sections of the palatal plates 
of C. obliquus; and on comparing them with the figure of the 
“teeth and a small portion of the jaw” of the so-called 
Saganodus, no difference of the slightest importance can be 
perceived. The six wedge-shaped ridges seen in transverse 
section stand up from the bony network of the plate in the 
form of conical tooth-like processes, all inclined a little to one 
side, and increasing in size towards the same side, and having 
their reticulated substance continuous with that of the plate. 
In all these respects the resemblance to the figure is so great 
that no one can doubt for a moment that the so-called jaw 
and teeth of Saganodus are identical with the palatal tooth of 
one of the Ctenodi. 
In the example figured by Prof. Owen, as also in many of 
our specimens, the external enamel and the peripheral walls 
of continuous matter have been worn away. His section is 
evidently a little diagonal, as proved by the increased depth 
of the ite (“jawbone”). And the minute structure, as ren- 
dered in fig. 3, is perfectly similar to that of many of our 
specimens. 
_ In the so-called Saganodus we see a remarkable example of 
the danger of trusting entirely to sections of minute objects, 
the planes of which are not understood. The oral armature of 
Ctenodus we have seen is composed of plates having on the 
surface several transverse wedge-shaped ridges, which are 
usually denticulated or tuberculated. Had it been understood 
that the specimen examined was a section cutting such ridges 
transversely, it never could have been described as a fragment 
of a “jaw supporting conical teeth.” 
It has been already stated that the enamel is frequently 
