652 
PROFESSOR WILLIAMSON ON THE STRUCTURE AND 
is much greater than usual. On inverting one of the scales, and examining these 
laniellee by means of transmitted light, we perceive that the calcareous granules are 
arranged on a more uniform plan than in the examples previously described ; they are 
not only in layers, but the individuals of each layer have a tendency to follow one 
determinate direction, being arranged with their longer axes parallel to one another. 
Fig. 10 represents the appearance presented by a horizontal section of this portion 
of the scale. When highly illuminated, the membranous tissues become so transpa- 
rent as to be almost invisible, and the distribution of the calcareous granules then 
becomes very obvious. Those occupying the lowest plane when the seale is in situ 
(but which the inversion of the scale brings the nearest to the object-glass), appear 
as innumerable minute and almost invisible granules. As we successively bring the 
superior laminae into focus, we observe that the granules composing each layer be- 
come progressively larger as we penetrate the scale. When isolated, they ordinarily 
exhibit an obtusely fusiform contour, but as they grow in size they frequently become 
confluent. The increase in their size is produced partly by the confluence of pre- 
viously detached individuals, and still more generally by the addition of new layers 
of calcareous matter round their exteriors. Indeed the latter process has apparently 
been the direct cause of the former condition. 
In fig. 10, three such layers are represented as they appear in the inverted scale: 
10a is one of the lower series; 6 is a layer of larger granules, some of which {V) 
have become confluent ; and at c we observe a faint indication of a still higher series, 
which when brought into view are found to have coalesced, forming in faet the inferior 
surface of the middle tissue of the seale. This representation also shows, what I have 
already alluded to, that whilst all the granules composing one layer exhibit a uniform 
tendency to arrange themselves in one direction, those of different layers cross each 
other more or less diagonally. Bearing in mind the arrangement of the primary fibres 
of which these iaminse of membrane consist, it seems probable that the direction 
of these fibres influences that of the calcareous granules. 
As we proceed horizontally from the centre to the periphery of the scale, we find 
a similar change to occur as when we pass from the upper to the lower layers. After 
a certain time, the additions to the inferior surface of the calcareous layer almost 
cease to be made in the centre of the scale ; at all events, they do not continue to be 
developed as rapidly as in an earlier stage of growth, and as they still are nearer the 
margin of the scale. Hence many of the laminae which have become caleified to- 
wards their periphery, continue to be membranous in their central portions, or, at 
the most, only contain a few detaehed granules ; these however are often of a large 
size, and by their successive junctions with contiguous granules lose their fusiform 
contour and become more or less square or cuboid ; in this state they are manifestly 
the objects noticed by M. Agassiz, ‘‘sous la forme d’ovales ou de carres a contours 
orid)res et indistinctement limites,” and which he mistook for accidental lesions which 
separated his superior from his inferior tissue. 
