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XXIII. On the Microscopic Structure of the Scales and Dermal Teeth of some 
Ganoid and Placoid Fish. By W. C. Williamson, Esq. Communicated hy 
Dr. Lankester, F.R.S. 
Received June 1, — Read June 21, 1849. 
A-T an early period after the invention of the microscope, the structure of the scales 
of fish attracted the notice of observers. At that time, little was known respecting 
the important group to which M. Agassiz has since applied the term Ganoid ; ” 
their attention was consequently directed to the other subdivisions, and especially to 
the “ Cycloid” forms ; the object aimed at being to account for the concentric circles 
on the surface of the scale, which had been noticed by Barellus in 1656*. Hooke 
touches upon them in his ‘ Micographia,’ published in 1667. Five years later, the 
accurate Leeuwenhoek submitted them to a careful examination, and concluded, 
according to M. Mandl, “ Qu’il se forme chaque annee, une nouvelle ecaille au dessous 
de I’ancienne, qui la deborde, de sort que Ton aperqoit sur I’ecaille le bord de I’an- 
cienne ecaille, et qu’on peut ainsi en comptant dans une section transversale le nombre 
des couches, determiner I’^e du poisson et le nombre d’^cailles accessoires, qui fer- 
ment I’ecaille enti^re'l'.” 
During a century subsequent to this discovery, but little new light appears to have 
been thrown upon the subject ; and though Mandl, in the memoir just quoted, cites 
the names of Reaumur, Roberg, Petit, Schaffer, Raster, Ledermuller and Brou- 
soNNET, as having directed their attention to it, they appear to have left it pretty 
much where they found it. 
During the present century, Heusinger, Kuntzmann, Ehrenberg, Agassiz, Mandl 
and Owen have in succession investigated the matter, but the labours of the three 
last alone require a more special notice. 
At an early period in the progress of the colossal labours of M, Agassiz, he was 
struck with the vast importance of studying the scales of fish, and, as is well known, 
ultimately made their variations the basis of his classification. In the fourth chapter 
of his large work, headed “ Dermatologie et en particulier des Readies des poissons:}:,” 
he enters very elaborately into the structure of the skin and scales, with the relation- 
ship of the one to the other ; and he especially seeks to illustrate, what was then a new 
topic, the structure of the scales of many of his ‘^Ganoid” fish, pointing out their 
enamelled {emailU) surface, the large development of a dentine-like substance in the 
* Petrus. Observationum Microscopicarum Centuria. 
t Annales des Sciences Naturelles, vol. ii. p. 338. I Poissons Fossiles, vol. i. p. 61. 
