from the Shahs of the Northumberland Coal-feld. 357 



his assertion tliat " la substance qui forme la surface exterieure 

 de la dent est parfaitement homog-fene, sans trace de structure 

 quelconque." 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 ohliquus (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 (pi. 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 it 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 G. ohliquus ; 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 inchned 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 plate ("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 teeith." 



It has been already stated that the enamel is frequently 



