452 MR. W. C. WILLIAMSON ON THE MICROSCOPIC STRUCTURE OF THE 
15^'; and which usually exhibit a tendency to run parallel with the long axis of the 
scale. 
In fig. 15 these are cut transversely, but in fig. 17, which represents a vertical 
section of a smaller species of MegaUckthys from the coal-measures near Leeds, the 
large canal is divided longitudinally, \7 g- In this latter species, the ramJfications 
of the Haversian canals are much less extensively developed than in M. Hihbertii. 
The same is the case with an analogous scale from the old red sandstone of Cromarty. 
With the exception of the thin superficial layer of ganoin, 15 h, all that portion of 
the scale which lies above the plane of the network of narrow tubes, 15 c, consists of 
a beautiful and largely developed form of kosmine; the remaining channels are 
Haversian canals, penetrating true osseous tissues. The two structures merge in each 
other through the ascending cul-de-sacs. 
Below this system of Haversian canals, we find a large development of exceedingly 
thin parallel laminae, the majority of which extend completely across the scale, fig. 15 i 
and 18 a, each having an average thickness of about ir sVo lh of an inch. Their 
number and regular parallelism are alike the greatest towards the central and ante- 
rior portions of the scale. As we approach the posterior margin, each one becomes 
thicker, less uniformly parallel, and more disposed to curve upwards, mingling with 
those forming, by their involutions, the Haversian canals. 
Inferior to these parallel laminae, which divide the scale horizontally into two por- 
tions, we find a second distribution of Haversian canals, fig. 15 A: and 18 They 
are especially developed along the centre and across the anterior extremity of the 
scale, where they form a projecting ridge. I have not found any example in which 
these canals penetrate the parallel laminae towards the centre of the structure to any 
considerable extent ; but as we approach the anterior margin, where the laminae lose 
much of their parallelism, a free communication is established between the upper and 
lower portion by means of large anastomosing canals. Along the inferior surface 
the outline is exceeding irregular, in consequence of the existence of the numerous 
open orifices of these canals, and where, as at fig. 15 /, incipient canals are in pro- 
cess of formation, like those already described in the scale of the Sturgeon. On the 
upper surface of the anterior portion of each scale, also, where from the juxtaposition 
of the upper fold of the soft integument no ganoin was needed, and consequently 
it did not exist, we find precisely the same structure, which is one reminding us most 
strongly of the aspect presented by the section of a human foetal bone. Through 
these open canals, blood-vessels have had free access to every part of the scale. 
Fig. 15 is taken from near the lateral portion of a scale, where the extension of the 
inferior system of Haversian canals is limited, but fig. 18 is from a section which 
cuts across the central ridge already described, and where the degree of their deve- 
lopment is well shown. In this specimen the ganoin has been accidentally re- 
moved. 
