198 THE AMERICAN MONTHLY [July 
the boundary edge of which forms the characteristic con- 
centric line. In those scales in which the concentric lines 
do not cross the furrows the outer layers split as they 
harden, the interstices being filled with the newly form- 
ed transparant matter, and this goes on during the whole 
of life. The thin flat scale of the eel,which must be search- 
ed for beneath the skin, as it does not project from the 
surface, is a very beautiful object. At first sight, when 
viewed through a 1-inch objective, it appears to be of a 
cellular character, but careful study with a t-inch and a 
little management of the illumination shows this appear- 
ance to be caused by isolated concretions of carbonate of 
lime set in a layer of the same. Similar concretions may 
be easily seen in several of the scales between the outer 
laminae .and the inner transparant layer. All scales are 
very beautiful ; some of them are still more interesting 
through being mounted as opaque objects. Viewed by 
polarised light they are of course charming.—J. Lucas. 
Fish scales make beautiful objects, when viewed with 
reflected light, the scales of sole being often exhibited in 
this way, with the light falling on them in such a man- 
ner as to show the comb-like teeth. As transparent ob- 
jects they can be examined with the spot lens or equiva- 
lent arrangement, and with polarised light either with or 
without a selenite plate. As opaque objects it is only ne- 
cessary to clean and dry the skin ; as transparent objects 
the skin must first be dried and then mounted in Canada 
balsam. The following is the classification suggested by 
Agassiz, though subsequently modified, as quoted in the 
“Micrographic Dictionary.” Scales enamelled: Ganoid 
fishes.—Those the skin of which is regularly covered with 
angular thick scales, composed internally of bone and ex- 
ternally of enamel. Most of the species are fossil, the 
sturgeon and bony pike being recent. Placoid fishes.— 
Skin covered irregularly with large or small plates or 
points of enamel. Includes all the cartilaginous fishes 
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