666 
PROFESSOR WILLIAMSON ON THE STRUCTURE AND 
the composition of these, and of the superincumbent tissues, consist wholly of pro- 
longations of the reticulated fibres, fig. 24 d, which here cease to be aggregated into 
bundles, lose their reticular distribution, become much contracted in size, and spread 
out, forming thin membranous laminae which are turned inM^ards, so as successively 
to invest the upper surface of the scale, and where, by their calcification, they form 
the large tubercles with which each is covered. Thus we perceive that amidst all 
their singular modifications, the scales of Ostracionts maintain a close conformity to 
the type prevailing amongst those of the ganoid order of M. Agassiz. 
Another question to be considered bearing upon these structures, has reference to 
their mode of growth. This however is one which continues to be involved in some 
obscurity. That the new laminae have successively invested the entire scale does not 
admit of a doubt. The origin of the new layers of fibres is a difficulty which affects 
the history of these scales in common, not only with those of other groups of fishes, 
but with all periosteal growths whatever. The additions to the second class of fibres 
forming the radiating bundles, appear to be made to their extremities, partly by 
the prolongation of the pre-existing fibres, and partly by the gradual incorporation of 
new ones. The latter process would of course be rendered necessary by the constantly 
enlarging area which they would have to occupy. On examining the line of junction 
between the margins of two contiguous scales, the large horizontally disposed bundles 
of fibres (26 c) may be seen projecting from the edge of each plate into the intervening 
space. This is obviously the place where in some way or another the additions to their 
length are made. In the inferior margin of several specimens I have observed an 
appearance like what 1 have represented at fig. 24 f, where an investing membrane 
appears to be raised from the lower surface of the scale, supported as upon arched 
pillars, by bundles of fibres, proceeding from the previously formed layer. This 
formation of the radiating fibres, prior to that of the horizontal layers through which 
they pass, is in accordance with what I have just described with reference to the 
homologous fibres at the margin of the scale. It is possible, that after they have been 
formed, the space enclosed within this series of expanded arches may have become 
filled up with the horizontal portions of each lamina; but this must still be regarded 
as a doubtful question. It is however obvious, that at the lateral margins, the cor- 
responding fibres, which now become horizontal, project free into the open space, 
their investment with the upturned and reticulated portions of the central horizontal 
laminee being apparently the result of a subsequent process. 
The formation of the uppermost part of the scale is evidently accomplished by the 
successive additions of thin larnellee to the surface, such laminee being continuous 
with those which ascend from below : this continuity is well shown by fig. 22. As 
the vertical series of reticulated fibres approach the surface, their areolae gradually 
become smaller and more circular and soon disappear altogether ; at the same time 
the fibres spread out to form very thin lamellie, which soon become calcified. The 
large canals, 20 h and 22 6, as well as the smaller tubes (22 a) entering from the sur- 
