232 report — 1846. 



exist in the ordinary or endo-skeleton of other vertebrata. The learned 

 Professor of Comparative Anatomy in King's College, London, who regards 

 this as "the more philosophical mode of considering them*," has briefly 

 stated the homologies proposed by the supporters of this view, viz. that the 

 opercular bones are gigantic representatives of the ossicles of the ear (Spix, 

 Geoffroy, Dr. Grantf): or that they are dismemberments of the lower jaw 

 (De Blainville, Bojanus), — a view refuted by the discovery of the compli- 

 cated structure of the lower jaw in certain fishes, which likewise possess the 

 opercular bones: he then cites a third view, viz. that they are parts of the 

 dermal skeleton ; " in short, scales modified in subserviency to the breathing 

 function ;" an opinion which Professor Jones correctly states that he derived 

 from my Lectures on Comparative Anatomy, delivered at St. Bartholomew's 

 Hospital in 1835, and which he adopts, although its accordance with his first 

 proposition is not very clear. I have subsequently seen reason to modify that 

 view, though it has received the sanction of the greatest ichthyologist of the 

 present day, M. Agassiz ; and, as I have since found, had presented itself so 

 early as 1826, under a peculiar aspect to the philosophical mind of Professor 

 Von Baer. In his admirable paper on the endo- and exo-skeleton, M. Von Baer 

 expresses his opinion, that the opercular bones are (dermal) ribs or lateral 

 portions of the external cincture of the head J. 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 §. In 1836, M. Agassiz propounded his idea of the relation 

 of the opercular bones to scales in a very precise and definite manner; 

 though, as I have elsewhere shown ||, the chief ground of his opinion is erro- 

 neous. 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 contraire, 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 l'opercule, le sub-opercule, et* 



* Professor Rymer Jones, General Outline of the Animal Kingdom, 8vo, 1841, p. 509. 



t Lectures, Lancet, Jan. 11, 1834, p. 573 ; Outlines of Comp. Anat. p. 64. 



J " In mancher Beziehung gehbren die Kiemendeckel zu ihr, und ich halte sie um so 

 mehr fur (Haut) Rippen, d. h. fur Seitentheile der aussern Ringe des Kopfes, da ich sie auch 

 in den gewohnlichen Knockenfischen fur nichts anderes ansehen kann. Hat bei diesen auch 

 der oberste Knochen des Kiemendeckels wenig Aehnlichkeit mit Rippen, so geht dagegen 

 der unterste so uuverkennbar in die strahlender Kiemenhaut liber, das der Uebergang gar 

 nicht zu verkennen ist." — Meckel's Archiv, 1826, 3 heft, p. 369. 



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

 proposed by Geoffroy St. Hilaire (Annales des Sciences, t. hi. pi. 9), and Cuvier (Hist, des 

 Poissons, i. p. 232), and has been adopted by the learned Professor of Comparative Ana- 

 tomy in University College, who, speaking of the occipital vertebrae, 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. 543.) He subse- 

 quently, however, adopts and illustrates (p. 573) the homology of the opercular bones with 

 the ' ossicula auditus' of mammalia; and in the 'Outlines of Comp. Anat.' cites only the 

 Spixian and Blainvillian hypotheses (pp. 64, 65). In my Hunterian Lectures (vol. ii. 1836, 

 pp. 113, 130), 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 (haemal) arch ; not, however, of the occipital 

 vertebra, but of the frontal ; just as the branchiostegal rays are the appendages of the haemal 

 arch of the parietal, and the pectoral fins of that of the occipital vertebrae. That parts of 

 both endo- and exo-skeleton may combine to constitute the opercular fin is the more pro- 

 bable, inasmuch as we see the same combination of cartilaginous and dermal rays in the 

 pectoral fins of the plagiostomes, and in the median fins of most fishes. 



§ Urtheilen des Knochen und Schalengeriistes, fol. p. 122. 



|| Lectures on Vertebrata, p. 139. 



