138 LECTURE VI. 



ill subserviency to the breathing function ; an opinion which Professor 

 Jones acknowledges that he derived from my Lectures on Compara- 

 tive Anatomy, delivered at St. Bartholomew's Hospital in 1835, and 

 which he adopts. I have subsequently seen reason to modify that 

 view, although it has received the sanction of the greatest Ichthyo- 

 logist of the present day, M. Agassiz ; and although I find that, so 

 early as 1826, it had presented itself under a peculiar aspect to the 

 philosophical mind of Von Baer. In his admirable paper on the 

 endo- and exo-skeleton he expresses his opinion, that the opercular 

 bones are (dermal) ribs or lateral portions of the external cincture of 

 the head. * The idea of the relationship of the opercular flaps to 

 locomotive organs is presented by Carus, under the fanciful view of 

 their homology with the wing-covers of beetles and the valves of a 

 bivalve shell (i. p. 122.). In 1836, M. Agassiz propounded his idea 

 of the relation of the opercular bones to scales in a very pi'ecise and 

 definite manner ; though, as I shall presently show, the chief ground 

 of his opinion is erroneous. He says, — " Les pieces operculaires des 

 poissons ne croissent pas, comme les os des vertebres en general, par 

 irradiation d'un ou de plusieurs points d'ossification ; ce sont, au con- 

 traire, des veritables ecailles, formees, comme celles qui recouvrent le 

 tronc, de lames deposees successivement les unes sous les autres, et 

 dont les bords sont souvent meme denteles comme ceux des ecailles 

 du corps. Tels sont I'opercule, le sub-opercule, et I'inter-opercule. Le 

 supra-scapulaire meme peut-etre envisage comme la premiere ecaille 

 de la ligne lateral e, dont le bord est egalement dentele. Ou pourrait 



* " In mancher Beziehung gehoren die Kiemendeckel zu ihr, iind ich halte sle 

 um so mehr flir (Haut) Rippen, d. h. fiir Seitentheile der aussern Ringe des Kopfcs, 

 da ich sie audi in den gewohnlichen Knockenfischen filr nichts anderes ansehcn 

 kann. Hat bei diesen auch der oberste Knochen des Kiemendeckels wenig Aehn- 

 lichkeit mit Rippen, so geht dagegen der unterste so unverkennbar in die strahlen- 

 der Kiemenhaut iiber, das der Uebergang gar nicht zu verkennen ist. " (Mickel's 

 Archiv. 1826, 3 heft, p. 369.) 



An analogous idea of the relation of the opercular bones to the inferior or costal 

 arches is expressed by the learned Professor of Comparative Anatomy in University 

 College, who, speaking of the occipital vertebrfe, says — "The two external and 

 the two lateral occipitals form the upper arch, and the two opercular and two sub- 

 opercular bones constitute the lower arch." (Lectures, Lancet, 1834, p. 523.) He 

 subsequently, however, adopts and illustrates (p. 573.) the homology of the oper- 

 cular bones with the " ossicula auditus" of Mammalia; and in the "Outlines" 

 (xxviii.) cites only the Spixian and Blainvillian hypotheses (pp. 64, 65.'). I have 

 adduced the grounds which have led me to the conclusion that the opercular bones 

 are neither ribs of the exo-skeleton, nor inferior arches of the endo-skeleton, but 

 persistent radiating appendages of an inferior (h;cmal) arch; not, however, of the 

 occipital vertebra, but of the frontal ; just as the branchiostegal rays are the ap- 

 pendages of the hajmal arch of the parietal, and the pectoral fins of that of the 

 occipital vertebra. That parts of both endo- and exo-skeleton may combine to 

 constitute the opercular fin is the more probable, ina'^miicli as we see the same com- 

 bination of cartilaginous and dermal rays in the pectoral fins of the Plagiostomes, 

 and in the median fins of most Fishes. 



