DEVELOPMENT OF THE SCALES AND BONES OF FISHES. 
677 
are arranged in oblique vertical lines, which successively run out at the border, as 
they approach the upper and lower surfaces in the two halves of the section. In this 
respect, as well as in their somewhat elongated contour, they exactly correspond with 
the same areolse in the vertebrae of the common Dog-fish. Within these there exists 
a second series, 35 &, which occupy the greater part of the lightly-shaded margins in 
fig. 36. The latter areolae are small, almost square, and arranged in curious undulating 
chains, which proceed from the central mass of spherical cavities towards the concave 
articular surfaces, the various rows being separated by rods of calcareous matter. 
Not having been able to obtain the bones of Carcharias in a very young state, 
I cannot speak with absolute certainty as to the directions in whicli ossification 
has extended itself ; I suspect, however, that it has combined the leading features 
of both the Ray and the Dog-fish ; whilst, like the former, the greater portion of the 
new bone has been added to the exterior of the osseous cylinder, primarily formed 
round the chorda : there have also been some additions (35 a) made to its internal 
surface, especially at its two concave extremities, as in the case of the Dog-fish. The 
principal point about which I am in doubt, has reference to the way in which the 
four divergent osseous cones have been formed ; whether they have been directly 
developed from the osseous cylinder, as in the Ray, or whether their ossification has 
commenced at the surface, and been developed internally, until the whole of the 
subjacent cartilage has been incorporated, and they have thus become united 
throughout with the inner cylinder. It is most probable that the former of these 
modes has been adopted ; but the point can only be determined by an examination 
of the bones of some very young examples of the fish. 
On turning from the vertebrae to the jaws of the same fish, we obtain results which 
substantially coincide with those obtained from the Ray and the Dog-fish. The 
cartilage of the jaw has long been known to be invested by a layer of vertically 
elongated calcareous prisms. Their structure closely corresponds with that of those 
covering the vertebral and cephalic cartilages of the Rays, only instead of being 
merely flattened plates, a rapid vertical development has caused them to be more 
elongated ; but their internal areolae are arranged in much the same w^ay as those 
of the Thornback, and their formation has been preceded by a corresponding ar- 
rangement of the cartilage-cells, which their calcareous walls have invested. The 
jaws of Cestracion exhibit an identical structure. 
The several bones which enter into the composition of the rostrum of the Saw-fish, 
are also largely composed of an aggregation of similar small calcareous prisms. An 
imperfect sketch of a portion of this structure is represented in my last memoir*. 
I had noticed the difference between its aspect and that of ordinary bone, but not 
having then been able to interpret its meaning, I spoke doubtfully as to its osseous 
character. There is however no doubt that the entire rostrum which supports the 
lateral teeth consists of two kinds of bone, covered over with a fibrous skin in which 
* Philosophical Transactions, Part II. 1849, fig. 34 a. 
MDCCCLI. 4 S 
