436 MR. W. C. WILLIAMSON ON THE MICROSCOPIC STRUCTURE OF THE 
upper part of many of them, as well as the existence of a true osseous tissue with 
lacunae, and even Haversian canals in the lower portions of some scales. The usual 
existence of a structure more or less laminated, is accurately noticed ; and he espe- 
cially points out the close and striking resemblance between some of these organisms 
and the teeth of fishes ; one general result of his observations being a conviction 
that these scales were formed by the gradual and successive deposition of layers of 
osseous tissue, a practical revival of the opinion first promulgated by Leeuwenhoek, 
nearly two centuries ago. These views were opposed by M. Mandl, in a memoir^^ in 
which he endeavoured to account for the structure of cycloid scales especially in a 
totally different way. This publication elicited from M. Agassiz an effective reply, 
in which he gives the following valuable summary of his views : — “ J’envisage I’ecaille 
du poisson com me une secretion epidermoidale, absolument analogue a celle des ongles 
et autres de meme nature, qui s’observent chez les animaux superieures. Cornrne les 
ongles, elles se composent de lamelles tres fines d’une substance cornee, superposees 
dans fordre de leur formation. L’organe secr^teur est la poche epidermoidale dans 
laquelle elles sont enfoncees par leurs bords anterieurs. La portion de fecaille re- 
couverte par le feuillet superieur de cette poche est plus ou moins considerable ; le 
feuillet inferieur, au contraire, recouvre presque toute la face interne de fecaille, ex- 
cept6 dans quelques ctenoides, oh la face inferieure des dentelures est libre. Les 
lamelles nouvellement form^es, sont plus modes, mais de meme composition que les 
plus anciennes. La poche grandit a mesure que I’ecaille develope, de sorte que les 
lames, nouvellement deposees, sont toujours plus grandes que les anciennes. Les 
stries concentriques de la poche sont dues a cette circonstance, en ce sens que le 
bord de chaque nouvelle lame occasionne par le pression qu’il exerce sur la poche 
un pli, ou plutot une impression tr^s legere qui correspond naturellement au bord de 
cette lame. Les lignes concentriques des 6cailles sont le reflet des bords des lamelles 
superposees. Aussi sont elles plus nombreuses chez les poissons ages que chez les 
jeunes'f'.” 
In the body of his large work:|:, M. Agassiz introduces descriptions of the micro- 
scopic structure of some ganoid scales, especially of those of the recent Lepidosteus 
osseus and Polypterus, as well as of the Lepidotus gigas and L. unguiculatus : and in 
the volume on the fossil fish of the old red sandstone, he gives the result of some very 
careful examinations of the scales belonging to many of the interesting genera from 
that group of deposits. 
Professor Owen has slightly touched upon the subject in his recently published 
leetures§, where he observes, “ In the Lepidosteus, the scales defend the body in 
close-set oblique rows ; are thick, completely ossified, and with an exterior hard, 
shining, enamel-like layer, having the microscopic structure of the hard dentine of 
sharks’ teeth ; the subjacent osseous part exhibits the radiated corpuscles. I described 
* Annales des Sciences Naturelles, vol. ii. f Ibid. vol. xiv. p. 108. + Poissons Fossiles. 
§ Lectures on the Comparative Anatomy and Physiology of the Vertebrate Animals, 1846, p. 140. 
