466 
MR. W. C. WILLIAMSON ON THE MICROSCOPIC STRUCTURE OF THE 
is no material difference between these and the dermal teeth of the Dog-fish ; but 
beneath them, and imbedded in the soft tissues of the true skin, we find a vast number 
of small, irregular calcareous nodules, 33 d, each of whicli consists of a series of 
concentrically arranged lamellm. They contain neither lacunae nor visible tubes, 
but frequently exhibit small brown points, which however may merely be some effect 
of their subsequent mineralization. Though not composed of true bone, these are 
surely to be regarded as a rudimentary attempt at the extension, amongst the Placoids, 
of that calcareous exo-skeleton which has received so complete a development in the 
ganoid fish. 
We may now for a moment retrace our steps and endeavour to mark some of the 
successive stages in the development of this portion of the exo-skeleton. 
In the common Thornback, Raia clavata, Cuv., a long central row of dermal teeth 
extends from the head to near the extremity of the tail. They exhibit the tubular 
structure found in this class of objects, but contain very little calcareous matter ; they 
are scarcely more solid in their structure .than the cartilaginous column which they 
surmount. The skin of the same fish is studded over with still more minute teeth, 
but which contain much more earthy matter: we have however no trace of true bone. 
Each dermal tooth consists of a succession of conical lamellse placed one upon 
another ; the apex, which rises above the cuticle, resembles that of the Dog-fish in 
structure ; inferiorly, these lamellae expand into a stellate base, in which portion they 
are much less consolidated, considerable spaces occasionally existing between indivi- 
dual layers after they have been artificially dried. The pulp-cavity is quite open in- 
feriorly, there being no extension of the lamellae across its base, and consequently no 
necessity for the horizontal canals, which are wanting. In the shagreen of the Dog- 
fish we have an advance upon this structure. The lower tissues are more consoli- 
dated, and present an extension of the lamellae across the base, closing in the pulp- 
cavity as already described, and being only perforated by the narrow canals, fig. 32 d. 
In Hyhodus reticulatus we find dermal teeth of a similar type to those of the Dog-fish, 
but we have a further development of calcareous granules in the subjacent skin, but 
no true bone. In Macropoma we advance still further. In each scale we find a 
laminated texture, probably analogous in its nature to the expanded bases of the 
teeth in the shagreen of the Thornback : upon this texture, the teeth, no longer 
isolated, are aggregated ; whilst on the surface of the scale thus formed, we find, for 
the first time, a thin film of true bone. In the operculum of Macropoma, the sub- 
structure upon which the dermal teeth are implanted exhibits all the essential cha- 
racters of true bone, its laminated structures preparing us for the ganoid fish, where 
not only the operculum but also the other scales are of an osseous nature. Amongst 
these we still find scattered dermal teeth, studding the scales of Dapidius granulosus, 
presenting the same external contour, internal pulp-cavity, branching tubuli and 
canals communicating with the exterior as in the preceding forms. 
These successive steps, conducting us from the dermal appendages of the Placoids 
