DEVELOPMENT OF THE SCALES AND BONES OF FISHES. 
653 
As we leave the centre and proceed towards the circumference of the scale, its in- 
ferior membranous laminae successively enter the field of vision. These contain more 
numerous granules, but they are of smaller size. As we come nearer to the margin, 
the granides become still more numerous, but exhibit a corresponding diminution in 
their bulk, until, at the edge of the scale, the otherwise transparent membrane 
assumes a hazy aspect from the presence of the innumerable minute calcareous atoms 
which are diffused through its substance. 
Whilst the scale of the Perch thus supplies a beautiful illustration of the way in 
which the calcareous middle layer is gradually produced, it also explains the peculiar 
structure which induced M. Agassiz to constitute the Ctenoid a distinct group from 
the Cycloid order, distinguishing it by the teeth which fringe the posterior margin 
of the scale. 
Fig. 12 represents a small portion from the surface of the posterior part of the 
scale of the Perch, a being its marginal extremity. The marginal teeth {12 h, c) are 
long, pointed posteriorly and dilated anteriorly ; they are not raised vertically, like 
those on the scale of the Carp, but are depressed to the level of the surface of the scale. 
Behind each of these teeth we see that there exist numerous bases of similar teeth 
(12 e), which in the earlier stages of growth successively fringed the posterior mar- 
gin of the scale, but of which the apices appear to have been worn away prior to the 
development of a new series; the abraded extremity of each tooth abutting against 
the dilated and concave base of its successor in such a way as almost to give the im- 
pression of a true articulation. These teeth are arranged in an alternating order, as 
if, when each new marginal growth had taken place, new teeth had only been added 
to the front of every other perpendicular row ; thus the individuals b and c belong to 
different series, the former being apparently of a more recent formation than the latter ; 
the remains of those in the more central parts of each row exhibit the same arrange- 
ment. Fig. 12 d is the soft membrane within which these teeth are imbedded. 
Fig. 11 represents a vertical section of one of these perpendicular rows of teeth, 
showing their relation to the other tissues of the scale. 1 1 a is one of the unbroken 
marginal teeth, whilst h are the remains of those of earlier growth ; c is a thin mem- 
brane which invests the upper surface of all the teeth ; d is the peripheral portion of 
the lowermost of the membranous laminse of the scale, and on the surface of which, if 
not actually within its upper tissues, the calcareous tooth a has been developed ; e re- 
presents a superior lamina, in which the very minute granules forming the middle 
layer are beginning to appear. 
On tracing these marginal teeth back to the middle of the scale, we find that they 
are but modifications of its third or uppermost layer, thus corresponding with figs. 3 c 
and 4 c, being more especially the exact homologues of the tesselated hexagons seen 
in fig. 8, which, it will be remembered, represents a similar fragment taken from the 
corresponding portion of the large scale, fig. 1. 
The question which now suggests itself is, what relation does the superior invest- 
MDCCCLI. 4 P 
