450 MR. \V. C. WILLIAMSON ON THE MICROSCOPIC STRUCTURE OF THE 
plane of the line 12 e,f, the same letters are employed to mark the corresponding- 
portions of the two sections : thus, whilst the portion 14 e dips rather deeply into the 
lower part of the scale, the oppposite one, 14 f, cuts across the bases of the arches 14 ^ 
shortly after taking their rise from the body of the scale. We see that they do not 
spring up at right angles to the axis of the ridge, but obliquely ; the section of this 
portion of each intervening cavity, 14 c, being somewhat pyriform. Each lamina 
is about iwotli inch in thickness. Between these and the body of the scale is 
a series of anastomosing canals, lA d, which connect the isolated cavities together, 
opening into them by small oval orifices, 14 h. Traces of the corresponding canals 
are seen \n \ 2 d and 13 d, whilst their orifices are likewise shown in 12 h and 13 h. 
In the lower part of this section, 14 c, we find numerous laminse cut through some- 
what obliquely, 14 g. These are also seen in the two vertical sections. 
Singular as is tlie construction of this scale, a very careful investigation of it has 
satisfied me that it is formed on the type of those previously described. The pointed 
extremity of the section, fig. 13, represents the anterior margin of the scale, which has 
been covered with the fold of soft integument and by the free edge of the antecedent 
scale ; the curious arched plates, 13 b, are in reality formed by the upturned portions 
of the lamellae, and probably correspond with the laminae of the other scales ; each of 
them, whilst enclosing those previously existing, has not been deposited in immediate 
contact with it, but intervening spaces have been left ; whilst between each of these 
cavities there exist connecting channels admitting of a free vascular intercommunica- 
tion. The cavities and the canals together appear to be the representatives of the true 
Haversian canals ; the large cavities bearing the same relation to the connecting 
passages, probably, that the large cancelli of Mammalian bones do to the Haversian 
canals which open into them. Each of the arched plates contains numerous lacunae, 
with peculiarly long trailing canaliculi, especially at the surface of the scale. No 
appreciable layer of ganoin covers these arches. 
The modifications of the Lepidostean type of scale found in Aspidorhynchus wad Aci- 
penser, conduct us to some of the most complicated and beautiful structures that I have 
yet seen amongst the ganoid fish, occurring in the genera Megalichthys, Diplopterus, 
and Holoptychius but notwithstanding all their complications, it is not difficult to 
trace the same principle of growth which we have thus far seen to apply in every case. 
Megalichthys. — The matured scales of this genus exhibit on their exposed surfaces 
a layer of bright shining ganoin, which is densely covered with minute puncta. These 
were noticed by M. Agassiz, who says, respecting them, “ Ce sont des petits points 
creux, extremement rapproches, et dont les intervalles en relief forment un reseau de 
mailles*.” On making a vertical section of a scale of Megalichthys Hibbertii from 
the upper coal-measures of Lancashire, fig. 15, 1 found that each of these puncta con- 
stituted the orifice of a vertical trumpet-shaped cavity, 15 n, very narrow superiorly, 
but expanding and becoming triangular or quadrangular inferiorly. This is well sliown 
* Poissons Fossiles, vol. ii. part. 2, p. 154. 
