454 MR. W. C. WILLIAMSON ON THE MICROSCOPIC STRUCTURE OF THE 
facts is, that Mr. Quekett’s objection to the arrangement of the Lepidosiren amongst 
fishes, as proposed by Professor Owen, derived from the form of its lacunse, is not a 
valid one. 
On viewing the variety and complication in the arrangement of the elementary tissues 
combining to form the scale of Megalichthys, it is difiicult to resist the conclusion 
that it must have been constructed on a very different plan to that followed in the 
genera previously described. I am satisfied however that such has not been the case 
to any material extent. We have seen that the results arising from the successive 
organization of lamellee have gradually increased in complexity as we ascended from 
Ley'idotus and Seminotus to Gyrodus, Platysomus, Aspidorhynchus and Acipenser. 
This complexity appears to have reached its climax in Megalichthys, at least so far 
as refers to the fish that I have had an opportunity of examining. At the same time 
there are some points of detail which differ from those which I have observed in the 
genera already noticed, and consequently I would express myself with legitimate 
caution on the point. 
I believe, however, that some of the parallel laminae, fig. 15 / and 18 a, have been 
formed the first : whilst additional layers were being organized, inferiorly, by intra- 
membranous ossification through the agency of the lower portion of the secreting 
sac, these laminae being parallel to their predecessors, the corresponding and coeval 
portions, being secreted by the upper wall of the sac, were much undulated, their vari- 
ous inflexions laying the foundation of the Haversian canals. After this process has 
continued for some time, and a considerable amount of thickness been given to the 
scale, the new lamellae added to its inferior surface, instead of retaining their paral- 
lelism with those already formed, have begun to assume an undulatory arrangement, 
in the same way that all the superior ones, corresponding to the upturned lamellae of 
the Sturgeon, had done from the commencement. This inflexion of the newly-added 
lamellae did not take place to an equal extent over the entire inferior surface of the 
scale, but was chiefly confined to the centre and to one extremity ; the remaining 
portions, and especially that occupying the anterior margin, retaining their tendency 
to horizontal parallelism. 
The osseous framework of the scale being thus completed, new processes have come 
into operation. Prolongations from the periosteal membrane have lined -the Haver- 
sian canals, and these have deposited new and internal lamellae — at once thickening 
their walls and diminishing their diameter — a process, of which we have hitherto met 
with no trace in the scales of any of the fish described, excepting in the opercular 
bone of Lepidotus. 
At some period prior to this partial filling up of the Haversian canals, a deposition 
of kosmine has taken place on the surface of the scale, but of which, also, in the first 
instance, only the framework has been formed. The careful preparation and examina- 
tion of numerous sections has enabled me, I trust satisfactorily, to remove much 
of the obscurity that has hitherto rested upon this portion of the subject. 
