DEVELOPxMENT OF THE SCALES AND BONES OF FISHES. 
679 
was more internal ; its cells also are very much less conspicuous, though it is con- 
tinuous with the true cartilage, and merely a modified form of it. The external por- 
tions of the rostral cartilages of Pristis have undergone some similar but more marked 
modification, producing the peculiar differences that exist at the opposite extremities 
of each of the calcareous prisms. The external continuous layer is obviously a rudi- 
mentary form of membraniform bone. 
The existence of these peculiar plates and prisms of chondriform bone, which ap- 
pear to distinguish the recent sharks and rays from other fishes, is of some import- 
ance in its practical application to geology. In my memoir already so often referred 
to, I pointed out the circumstance, that in connection with what had always been 
regarded as the skin of the fossil Hybodus reticulatus from the lias of Lyme Regis, 
there existed a deep-seated layer of calcareous granules, which appeared to have been 
formed within the membranous tissues of the skin=^. The more extended examination 
of recent forms which I have since made, has given me reason to conclude that these 
granules have belonged to the surface of the cartilaginous skull. The disappearance 
of the membranous portion of the skin, and of the soft cartilage, during the process of 
fossilization, would cause the dermal teeth implanted in the former, and the thin layer 
of calcareous granules investing the surface of the latter, to come into contact, and 
be permanently preserved in that state of close contiguity ; so that what, along with 
other observers, I had always regarded as a layer of true skin, appears to be a com- 
bination of the calcareous elements of both the skin and the skull. On examining 
portions of the jaw in which the teeth are planted, and which is certainly a part of 
the endoskeleton, I find that it consists of a congeries of granules which are identical 
with those just referred to. In the memoir, I pointed out the existence, in the inte- 
rior of each granule, of numerous small brown points, as well as of concentric lines 
of growth ; the former of these I thought might possibly be merely the result of 
mineralization ; I have now however no doubt that they have once been true areolae 
formed by cartilage-cells, and that both these characteristics are homologous with 
the similar features presented by fig. 30. Some of the areolm appear to have been 
filled up with a yellowish substance, and are no longer to be distinguished from 
the rest of the structure, which now presents the same hue ; but the others have 
become occupied by brown carbonaceous matter, and are still obvious. We thus 
learn that the same structural peculiarities wdiich are exhibited by the skeletons of 
the recent sharks, have also existed in their fossil allies of the Liassic eera. 
But we can recede even still further into the past and obtain the same results. In 
the coal-measures of Leeds and Manchester, there are occasionally found small 
lustrous fragments, composed of a congeries of minute resinous-looking granules. 
When thin sections of these objects are examined under the microscope, we find that 
they have an internal structure closely resembling that of Hybodus reticulatus. 
Similar concentric lines of growth exist in each granule, following the outline of the 
numerous projecting points with which its surface is studded ; in the interior is 
* Philosophical Transactions, ut supra, p. 466, fig. 33. 
4 S 2 
