646 
PROFESSOR WILLIAxMSON ON THE STRUCTURE AND 
echancrures plus ou moins profondes du bord posterieur de la couche superficielle*/’ 
He observes others, however, which appear to be “ corpuscules propres, implantes 
sur la couche superieiire de I’ccaille ; ce sont des formations tout-a-fait analogues a 
la couche superieure et qui sont d^posees sur cette derniere, comrne les lignes et les 
bandes de la couche superieure le sont quelquefois sur la couche inferieure'f'.” 
It will be seen from the details about to be brought forward, that both these 
distinguished observers have fallen into considerable errors, which affect alike 
their respective interpretations of the structure, and of the growth of Cycloid and 
Ctenoid scales. There is no doubt that M. Mandl is quite correct in distinguishing 
an inferior portion, composed of fibrous lamellee, from the calcareous covering with 
which it is surmounted ; and also in describing the fibres of each lamella as being 
parallel to each other, whilst they traverse those of contiguous lamellse in a diagonal 
direction. But we shall find that no form of cartilage enters into the composition of 
these scales ; their upper portions contain no cells or cartilage corpuscles ; and so 
far from having been formed in the way described by M. Mandl, this superior cal- 
careous layer consists of two very distinct substances, which differ alike in their 
structure and in their genesis. As an inevitable result, the whole of M. Mandl’s 
hypothetical conclusions fall to the ground. 
On the other hand, M. Agassiz has failed to detect either the fibrous nature of the 
inferior layers, or the existence of two distinct structures in the upper or calcified 
part of the scale ; and whilst he has obviously seen the small calcareous granules 
which we shall find are distributed along the line of junction between the middle and 
lower layers, and which M. Mandl regarded as cells or cartilage corpuscles, he 
also has wholly mistaken their nature ; the consequence has been the formation of an 
hypothesis which is as little tenable as that of M. Mandl. 
The true structure of these thin scales can only be successfally studied by means 
of vertical as well as horizontal sections. The preparation of such sections from thin 
flexible scales, whose entire thickness is often not more than from ^^th to 
an inch, is, as I have learnt from painful experience, only to be accomplished after 
long practice in the preparation of microscopic objects. Neither is it a task that can 
be well delegated to the lapidary. If the observations are to possess value and trust- 
worthiness, the preparations must all be made by the observer himself; since an exact 
knowledge of the direction in which such sections traverse the scale is essential to 
the right comprehension and interpretation of the structures which they reveal. 
Without this knowledge all would be confusion and obscurity. Such sections may, 
however, be prepared even from the thinnest of scales. In all their essential charac- 
teristics, the Cycloid and Ctenoid scales are constructed upon the same plan. The 
differences which exist between them we shall find to be very trifling, and indeed 
some examples exist where it becomes difficult to determine to which of the two 
groups an individual scale belongs. 
* Poissons Fossiles, p. 73. 
t Idem, p. 73. 
