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At 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 ʻMicographia,’ 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 année, une nouvelle écaille au dessous de l’ancienne, qui la déborde, de sort que l’on aperçoit sur l’écaille le bord de l’ancienne écaille, et qu’on peut ainsi en comptant dans une section transversale le nombre des couches, déterminer I’âge du poisson et le nombre d’écailles accessoires, qui forment l’écaille entière.” 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, Schæffer, Baster, Ledermuller and Brousonnet, as having directed their attention to it, they appear to have left it pretty much where they found it.

XXIII. On the microscopic structure of the scales and dermal teeth of some ganoid and placoid fish