648 
PROFESSOR WILLIAMSON ON THE STRUCTURE AND 
not been developed between two of the membranous layers, their development 
causing the latter tissues to diverge, but are the result of a calcification of the 
membranous laminae, which they incorporate into their structure. They commence 
as a small calcareous atom, and increase in size by the external addition of new 
concentric laminae, the direction of the latter not being parallel with, or having any 
reference to that of the laminae of fibrous membrane which they so amalgamate ; 
thus they are not depositions from, but growths in the membrane ; which growths, 
as they increase in size, retain their primitive tendency to assume a lenticular 
form. 
On carefully examining the middle layer of the scale (figs. 3 h and 4 b), we per- 
ceive that it is chiefly composed of an aggregation of snch small lenticular bodies, 
which appear to coalesce as they increase in size ; the newer calcareous additions 
sometimes lose their lenticular contour from having to enclose several lenticular 
granules which have so coalesced. This is seen at fig. 4 e. 
As we approach the margin of the scale, we perceive that this middle layer gra- 
dually diminishes in thickness until it becomes reduced to a few scattered calcareous 
granules, like those which remain isolated in the membranous portion of fig. 3 ; thus 
this calcareous layer is not only thicker, but is more consolidated towards the centre 
of the scale than near its margin. On becoming thus confluent and consolidated, 
these granules assume a new aspect. The layer, thus formed, splits up into hori- 
zontal laminee, which correspond, as to their direction, with that of the membranous 
laminae prior to their calcification ; the latter having apparently influenced the di- 
rection of the former. They also exhibit numerous small vertical subdivisions 
disposed at irregular intervals. 
On examining a horizontal section passing through these secondary calcareous 
laminae, we meet with an appearance like that represented by fig. 6. The outlines of 
the small lenticular granules are still visible (fig. Gn), presenting varying degrees of 
distinctness. But in addition to these, the section is also traversed by numerous 
small and slightly curved fissures ; the principal ones in each lamina (fig. 6 h) have 
a decided tendency to run in one direction, which, as in the case of the fibres of the 
membranous portion, is different in two contiguous portions. Minor fissures also 
exist, either fringing the larger ones at an acute angle (6 c) or forming an additional 
series, arranged at right angles to those which constitute the principal group. 
In some instances these fissures appear to take their rise from primary ones which 
pass through the centre of many of the lenticular bodies (fig. 3 d) ; but more frequently 
they commence in small open spaces left between these bodies, which, when the 
specimen is mounted in Canada balsam, are seen to contain air. These fissures ob- 
viously correspond with the small vertical ones which intersect the calcareous laminae 
in the section fig. 4 e. Fig. 6 exhibits their horizontal arrangement as seen in three 
contiguous laminae, when viewed by transmitted light. 
The third or uppermost layer (figs. 3 c and 4 c) is very different from the pre- 
