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. Grant+): 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{. 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 piéces operculaires des poissons ne croissent pas, 
comme les os des vertébres en général, par irradiation d'un ou de plusieurs 
points d’ossification; ce sont, au contraire, des véritables écailles, formées, 
comme celles qui recouvrent le tronc, de lames déposées successivement 
les unes sous les autres, et dont les bords sont souvent méme dentelés 
comme ceux des écailles du corps. Tels sont l’opercule, le sub-opercule, et 
* Professor Rymer Jones, General Outline of the Animal Kingdom, 8vo, 1841, p. 509. 
+ Lectures, Lancet, Jan. 11, 1834, p. 573; Outlines of Comp. Anat. p. 64. 
t “In mancher Beziehung gehoren die Kiemendeckel zu ihr, und ich halte sie um so 
mehr fiir (Haut) Rippen, d. h. fiir Seitentheile der aussern Ringe des Kopfes, da ich sie auch 
in den gewohnlichen Knockenfischen fiir nichts anderes ansehen kann. Hat bei diesen auch 
der oberste Knochen des Kiemendeckels wenig Aehnlichkeit mit Rippen, so geht dagegen 
der unterste so unverkennbar in die strahlender Kiemenhaut iiber, 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. iii. pl. 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 vertebrz, 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 auditis’ 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 (hemal) arch ; not, however, of the occipital 
vertebra, but of the frontal ; just as the branchiostegal rays are the appendages of the hamal 
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 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. 
