650 
PROFESSOR WILLIAMSON ON THE STRUCTURE AND 
are not only more definite in their contour, but become isolated, and at the extreme 
margin assume the form of small, flat and regularly arranged hexagons. Their ap- 
pearance is shown in fig. 8, which represents a fragment removed from this part of 
the scale, a being* its posterior and h its anterior portions. 
This uppermost layer covers the entire scale, even to its extreme periphery. In 
this respect, as well as in its internal structure, it differs from the subjacent calcareous 
portion. The latter becomes thinner, less consolidated, and finally ceases to exist at 
some little distance from the margins, as seen in fig. 4 ; on the other hand, the former 
exists on the upper surface of each of the membranous laminae, as their margins suc- 
cessively rise towards the surface of the scale ; and even at the periphery, where but 
one such lamina occurs, it is still covered over by the same superficial structure. 
This is a circumstance of importance, as tending to confirm the distinctness of the 
two calcareous structures. Though the way in which the superficial layer has in- 
creased in size is not very clear in this example, there are evidences, not only that 
it possesses a laminated structure, but that the lamellse constitute so many additions 
that have been successively made to its upper surface. The examination of other 
scales, shortly to be noticed, will be seen to confirm this conclusion. 
In the essential features of its internal structure, the scale now described may be 
received as a type of all the Cycloid and Ctenoid scales which I have examined. The 
increase in its size has evidently been accomplished by the addition of successive 
membranous lamellae to the inferior surface of those previously formed ; each new 
layer being rather larger than its predecessors. It is also obvious that the middle 
layer is produced by the formation and coalescence of the small lenticular bodies 
through the agency of which the calcification of the membranous laminae is effected- 
This calcification permeates the entire extent of the upper and earlier-formed lamellae, 
whilst with the exception of a few isolated granules, it has been confined to the mar- 
gins of those which are inferior and of more recent growth ; a distinction which will 
be readily comprehended on referring to the arrangement of these laminae as seen in 
fig. 2. The scale of the Carp {Cypriniis carpio, Linn.) affords a beautiful additional 
illustration of the true nature of these structures, and more especially of the upper- 
most layer. A small portion of a vertical section of this scale, made in a direction 
parallel to the lateral line of the fish, is represented by fig. 9. The lower series of 
membranous laminae are seen at 9 a ; 9 5 is the middle calcareous layer, and 9 c the 
superficial one. Tlie lower membranous laminae are arranged in precisely the same 
way as in the preceding scale, and the middle layer (9 b) only differs in the smaller size 
of the calcareous lenticular granules and in their being closely aggregated along the 
calcifying surface 9 c/, instead of being also scattered through some of the lower mem- 
branous laminae. After their coalescence and consolidation they break up, as before, 
forming a secondary stratification parallel with the original direction of the mem- 
branous layers ; and when the inferior surface is viewed by transmitted light, it 
exhibits appearances which closely correspond with those seen in fig. 6. 
