470 MR. W. C. WILLIAMSON ON THE MICROSCOPIC STRUCTURE OF THE 
that have been left out^ during the deposition of the membranous lamellae and their 
subsequent impregnation with calcareous matter*. 
The appearances presented by the scales described in the preceding pages, go far 
to confirm these views. I have not been able to detect the slightest trace of cartilage 
cells in either the recent scales of the Sturgeon and Lepidosteus, or in the dermal 
teeth and plates of any of the Piacoid fish. The decalcified scales of the Sturgeon 
and Lepidosteus, show that the lime is deposited in a granular form, in the minute 
interstices of membranous lamellte, and that, consequently, the origin of the magni- 
ficent lacunas with their largely developed canaliculi, must be explained without 
having recourse to the intervention of cartilage cells, either in producing the cavity 
or influencing its position in the bone. 
In the same way, the gradual progression to be observed as we pass from the simple 
laminated scale of Seminotus and Lepidotus, to the complicated development of 
Haversian canals existing m Acipenser, Holoptychius and illustrate how 
the periosteal laminae thickening the exterior of the shaft of a mammalian bone, may 
have been twisted and inflected; their undulations producing, first, grooves on its 
surface, and subsequently canals, from the arching over of the grooves so formed, 
by the corresponding but inversed inflexions of the more newly-formed lamellae : 
whilst in some cases, as in Mepalichthys, we find, what is less evident in the Stur- 
geon, that after these canals have been thus constructed, additional and more com- 
pletely concentric lamellae have been deposited within each canal, at once diminishing 
its diameter and thickening its walls. 
We also obtain an additional illustration of what the study of comparative anatomy 
so frequently reveals, viz. the unequal degree in which the various portions of an 
organized structure are developed, in reference to the homological type of each. 
Thus, whilst in Lepidosteus osseus we have one of the simplest forms of the laminated 
scale, associated with vertebrae exhibiting the ball-and-socket-joint of the Ophidians, 
and teeth approximating to a Saurian form, in Megalichthys and Holoptychius we 
have scales mainly consisting of a complicated arrangement of Haversian canals, and 
abounding in long fusiform lacunae, of the true reptilian type ; whilst the vertebrae of 
Holoptychius, certainly, and I believe of Mepalichthys also, present the double con- 
cave articulation ordinarily found in fishes and enaliosaurs. 
The further carrying out of this investigation into the microscopic structure of the 
scales of fish, will afford an important means of distinguishing different fossil species, 
and also, when prudently employed, of establishing their affinities and alliances; but 
at the same time I would venture to caution the palaeontologist against expecting too 
much from it. In many cases it will enable us to decide that two imperfect frag- 
ments belong to distinct species, and also to form a pretty correct judgment as to the 
general nature of each; but in numerous other instances, even very different genera 
* Dr. Quain’s Anatomy, 5th edition, hy Dr. Sharpey and Mr. Quain, part 2, p. cxxxii. et seq. 
