654 
PROFESSOR WILLIAMSON ON THE STRUCTURE AND 
ing membrane (11 c) bear to the inferior fibrous portion (11 d)} We have already seen 
that all the scales just described require the existence of a superficial membrane to 
render the growth of their superior calcareous layer intelligible. This we found to 
be especially the case in the scale of the Carp, though the latter is but a highly de- 
veloped example of the common type. In that of the Perch we have demonstrative 
evidence of its existence. Whether it is, in the first instance, merely the upper portion 
of a lamina (11 d), within the substance of which the teeth a and b are developed, or 
whether it is an injlected prolongation of the same lamina, modified in its structure, 
and which, after turning round the apex of the tooth, 1 1 «, is extended over the 
entire surface of the scale, I am unable to say. Analogical reasoning, based upon the 
structure of the ganoid scales, would lead us to the latter conclusion, in which case 
each new lamina of membrane, though apparently added to the inferior surface of the 
scale only, would really enclose the entire structure like a capsule. If, on the other 
hand, the former be the real interpretation, it is obvious that the upper surface of this 
superficial membranous structure must possess the power of perpetuating its existence 
by continuing to be developed superficially, as its /oicerfibres become gradually calcified 
and amalgamated with the substance of the uppermost calcareous layer. Be the pro- 
cess of its genesis what it may, we have here demonstrative evidence of the existence 
of such a superficial film of soft membrane as is essential to my hypotheses, account- 
ing for the peculiar structure and growth of the uppermost layer. The production 
and growth of fibrous tissues has long been one of the most perplexing subjects in 
physiological science ; and the peculiar forms of fibrous tissue existing in these 
scales, certainly do not diminish the difficulties with which the subject is already in- 
vested. Nothing here indicates a cellular origin of the fibres. 
In the scale of the Grey Mullet {Mugil CapHo, Cuv.), the posterior marginal teeth, 
though also pointed, are much shorter and broader, verging even to a rhomboidal 
form. Their persistent bases, as seen at a little distance from the margin of the scale, 
have received new calcareous additions to their upper surface, in the form of a crest 
of minute mammillee, which are especially evident at the posterior border of the new 
growth. On examining the anterior margin of the same scale, we see that the con- 
centric lines common to nearly all cycloid and ctenoid scales, are smooth at the 
periphery, whilst at a little distance from it, they also have received additions of the 
same mammillary character. These additions are the most obvious along the raised 
ridges which produce the tooth-like projections seen in fig. 3, only in the Mullet, as 
in the Perch, these ridges are much more acute, corresponding in this respect to 
the small transverse crenulated subdivisions of fig. 7- In these fishes, also, the conti- 
nuity of these ridges is only interrupted by a small number of the grooves radiating 
to the anterior margin of the scale, so many of which exist in the Mexican ex- 
an)ple, 7 «• 
An important homological question now suggests itself : wnat relation does this 
third tissue bear to those existing in the ganoid scales especially of Sauroid and 
