468 
MR. W. C. WILLIAMSON ON THE MICROSCOPIC STRUCTURE OF THE 
This supposed analogy is, in some degree, supported by the resemblance in the pro- 
cess by which the areolae of Megalichthys and the dermal teeth of the Placoids are 
developed and increased. In both examples it is by the addition of new internal 
layers around the central cavity. The same is the case in the tubercles and teeth of 
Macropoma. These facts (if correctly interpreted, and I believe them to be so) con- 
firm the necessity of my proposed restriction of the terms ganoin, enamel {dmail), &c. 
Two perfectly distinct structures have hitherto been comprehended in the expression 
enamel,” as hitherto applied to the scales of fish, viz. the superficial, transparent, 
hyaline tissue, which usually gives glossiness to the surface of the scale, and a sub- 
jacent one, which I propose to distinguish by the name of kosmine^. In some genera, 
such as Megalichthys, this latter structure is gradually blended with the former, the 
line of demarcation not being visible ; whilst in others, such as Lepidotus, Palceoniscus 
etc., it is perfectly distinct from it, blending rather with the subjacent osseous 
tissue. These two appear to be as distinct as bone and dentine. The ganoin exhi- 
bits no visible trace of structure beyond its arrangement in the form of laininse, and 
the occasional existence of minute coloured granular points. When separated from 
the tissue upon which it rests, it evinces a marked disposition to crack and splinter in 
every direction. The kosmine, on the other hand, in a fossil state at least, is usually 
coloured, and always exhibits some arrangement of minute branching tubes, resem- 
bling those of dentine ; and as, in some species of fish. Prof. Owen has pointed out 
the direct passage of Haversian canals into the pulp-cavities and dentinal tubes of 
the true teeth, so in the kosmine do we find a direct extension of the similar canals 
into the corresponding tubular structures of the surface of the scale. 
I have seen no instance in which this kosmine has been present without a covering 
of ganoin, whilst the latter may frequently be present without any subjacent kosmine. 
I am further led to conclude, that, whatever name be ultimately employed to repre- 
sent what I have designated kosmine, it must also be applied to those dentine-like 
tissues, which, in the form of dermal teeth, ornament the skins of so many Placoids. 
If then I am correct on these points, we must come to the conclusion, that, whilst 
the scales of many of the so-called ‘‘Ganoid ” fish, such as the Sturgeon, and other 
similar forms, exhibit few or no traces of either ganoin or kosmine, many of the 
“Placoids” exhibit such an extensive development of both, as finds few parallels 
amongst the Ganoids ; so that, not only have we several connecting links merging 
tlie two groups in one another, more of which links doubtless remain to be dis- 
covered, but the distinction of “ Ganoid,” as the term has hitherto been applied, 
ceases to be a physiological one. 
\n Megalichthys edich cul-de-sac communicates with the subjacent tissues through the medium of the Haversian 
canals, so in the Saw-lish the descending canal, 34 f, communicates with the soft integument, b, which alone 
separates it from the curious cancellated structure, a, representing the bony (?) part of the snout. It is obvious 
that we only require the upper and lower disc-like expansions of contiguous teeth to become confluent, to give 
us a structure closely resembling that which covers the bony scales of Megalichthys. 
* See page 442. 
