DEVELOPMENT OF THE SCALES AND BONES OF FISHES. 
667 
face^, are formed, not by any prolongations of cells or cell-nuclei, for none such exist ; 
but are the result of small apposite apertures left out in each successive layer as it is 
added to the preceding ones. 
I have examined scales from several other examples of Ostraciont fish, and find that 
they are all constructed after one common type, varying only in the extent to which 
the process of calcification has affected the membranous tissues. In one small spe- 
cies the scale is almost wholly membranous. A thin superficial layer of kosrnine, into 
which the tubes enter from the external surface, rests upon an equally thin homoge- 
neous layer of calcareous substance, which has evidently been formed by the aggre- 
gation of minute granules, not unlike those of a Cycloid scale. Below these, near 
the margin, one single, very thin vertical lamina unites with an equally thin hori- 
zontal one, extending from side to side ; whilst a third portion proeeeds downwards 
from the junction of the former two, along the line 19 d — to the lower edge of the 
scale. 
The imperfect development of the calcareous elements of the latter scale contrasts 
strongly with that of the kosrnine in some other portions of the fish. Three very 
large dermal spines project from its dermal covering, like those which give the name 
to Ostracion cornutus •, one of these proeeeds forwards from the upper anterior angle, 
immediately above and between the eyes, whilst the other two occupy the respective 
posterior inferior angles on eaeh side of the tail. Throughout the greater portion 
of their extent they are cylindrical, and have their surfaces ornamented with numerous 
longitudinal ridges and intervening sulci. At their bases, where they are depressed 
vertically, the ridges terminate in rows of small tubercles, like those which stud the 
surface of the scale. 
The internal structure of this spine is highly interesting, partly from the fact that 
it is merely one of the ordinary scales in which the superficial element is dispropor- 
tionally developed and drawn out longitudinally, and partly because it explains the 
true nature of some well-known fossil ichthyolites. It contains a hollow pulp(?)- 
cavity, which exhibits no appearance of having been occupied by a cellular pulp, but 
is partly filled by a membrane composed of reticulated fibres, like those found in the 
ordinary scales. Around this is placed the calcareous portion of tiie spine, which 
wholly consists of kosrnine, and includes the homologues both of the horizontal canals, 
20 b, and of the more superficial tubuli, 20 a. The former of these are seen in the 
thicker portions of the spine, running parallel with the pulp(?)- cavity, and give off 
small kosrnine tubes ; the latter enter from the outer surface. 
On making a transverse section, we obtain an exact fac-simile of the corresponding 
seetion of the fossil Ccelorhyncfnis, figured in my last memoir*. The superficial lon- 
gitudinal ridges are seen to be the outer edges of long plates radiating from the centre 
of the spine to its circumference, separated by thin intervening layers of reticulated 
kosrnine canals, which anastomose at their inner extremities with those which run 
* Philosophical Transactions, 1849, tab. 4-3. figs. 35, 36, 37. 
