SCALES AND DERMAL TEETH OF SOME GANOID AND PLACOID FISH. 451 
in Plate XLII. fig. 16, which represents a horizontal section of the upper surface of the 
same scale, as seen when looking downward through the transparent ganoin. The in- 
ferior portion has been more freely ground away at the light-coloured extremity h, than 
at the opposite one, where the section retains some of the deeper osseous layers ; con- 
sequently these vertical cavities are here cut across at their narrowest part and exhibit 
a circular contour, 16 a, but in the middle portion, we find that the section has passed 
through these cavities lower down, where they assume a triangular or quadrangular 
form, 16 5. In their upper and middle part, we not unfrequently see them to be 
surrounded by concentric rings, reminding us of the rudimentary Haversian canals 
in the opercular bone of Lepidotus semiserratus. After descending a little distance, 
these cavities give off three, four, or five narrow horizontal tubes, which commu- 
nicate with contiguous cavities, 15 c; thus combining to form a horizontal network 
which lies a little below the superficial ganoin. Neither these cavities nor the tubes 
into which they thus subdivide give off any minute branches. They are obviously 
but the channels of communication which lead to more important tissues. After 
giving off the tubes, the cavity becomes suddenly constricted, and, descending a little 
further, connects itself with a second and more irregular network of larger canals, 
15 and \Q d, constituting the uppermost of the Haversian canals. The meshes of 
the network of tubes 15 c, 16 c, constitute a series of cup-shaped areolar spaces, 15 e 
and 16 e*; into each of which the second layer of canals sends up an ascending 
branch, or cul-de-sac, like the stump of a pollard willow, 15^, \Qf. This gives off a 
multitude of ramifying tubuli, the main branches of which ascend, and distribute 
their terminal ramifications immediately under the ganoin. Their distribution is 
very well seen in the horizontal section 16, in the thicker extremity of which the 
branches are still connected with the cul-de-sac from which they spring, IQf, whilst 
at the opposite end we have only the branches or their terminal twigs remaining, 
16 A, the cul-de-sac being wholly ground away. The inferior portion of each areola 
is supplied with exceedingly minute recurved tubuli, which spring from the same 
point as the larger ascending branches. 
In each of these areolar systems, four or five of the branches, instead of subdividing 
until they become wholly lost, retain their original calibre, and connect themselves 
laterally with corresponding branches from adjoining areolae, forming a third network 
(16g, 17 5) which is still nearer the ganoin than the other two, and which gives off 
numerous minute horizontal and ascending twigs. 
After thus giving rise to these three well-marked systems of reticulations, distri- 
buted in the plane of the surface of the scale, the trumpet-shaped cavities continue 
their downward course, when they become lost in an irregular network of Haversian 
canals, 15 5, which generally terminate inferiorly, in others of a much larger size, 
* In the large scales of M. Hibbertii, each of these areolae have an average diameter of H-g-th of an inch. 
In the smaller species from Leeds, fig. 17, they average about 
