SCALES AND DERMAL TEETH OF SOME GANOID AND PLACOID FISH. 471 
of fish may present but a small and inappreciable amount of difference in the micro- 
scopic structure of their scales ; thus, for example, a longitudinal section of the scale 
of a Gyrodm exhibits a much closer apparent analogy to that of an Aspidorhynchus 
than to that of the more closely allied genus Platysomus ; though a close typical 
resemblance assimilates it to the latter. The uncertainty of this result is also in- 
creased by the various effects produced by the mineralization of the fossil scale. I 
have examined scales from some specimens of Lepidotus semiserratus, in which I 
had the greatest difficulty in detecting traces either of the beautiful arborescent tubuli 
or of the lacunae shown in fig. 4, they having been almost, though not altogether, ob- 
literated by the process of fossilization, teaching a lesson of caution, which the student 
will do well to remember. But notwithstanding these difficulties, the value of the 
inquiry, as furnishing us with an instrument which will facilitate the identification 
of affinities, is considerable, provided it is not made the sole standard of classification, 
but employed in conjunction with an equally minute examination of every other por- 
tion of the animal organism. 
It now only remains for me to acknowledge the great kindness with which Sir 
Philip M. De Grey Egerton, Bart., M.P., Dr. Mantell, Mr. Binney, Mr. John 
Edward Gray and Mr. Searles Wood, have afforded me every assistance in their 
power, by supplying me with many important specimens for examination which my 
own cabinet did not contain. To each of these gentlemen my warmest thanks are 
due for their most valuable cooperation. 
APPENDIX. 
vSince the preceding memoir was placed at the disposal of the Royal Society, the 
continued kindness of Sir Philip Egerton has enabled me to examine specimens of 
the curious premaxillary bones of the Coelorhynchus, obtained both from the London 
clay and from the tertiary beds of North America. They exhibit a form of kosraine 
which is alike new and interesting. I learn from Sir Philip Egerton that the rostral 
appendage of this fish is made up of two semi-cylindrical bones (see fig. 35 a a) in- 
closing a canal, which is double towards the base, but which becomes coalescent as 
it approaches the apex, of which latter portion, fig. 35 represents a transverse sec- 
tion. These bones are marked externally with longitudinal grooves and correspond- 
ing ridges, the latter being the external margins of a series of long cuneiform plates, 
whieh radiate from the centre to the circumferenee. 
Fig. 36 exhibits a profile representation of three of these plates, showing their in- 
ternal structure. Each plate is separated from its neighbour by a thin vertical net- 
work of small canals, fig. 36 a, which open, externally, along the narrow groove, 
fig. 36 h, separating the convex ridges. In the transverse section, the divided orifices 
of the canals constituting this network are visible, fig. 36 c, along the vertical line of 
demarcation between the segments. From each side of this line, vast numbers of 
3 p 2 
