ae ee 
ON THE VERTEBRATE SKELETON. 233 
Yinter-opercule. Le supra-scapulaire méme peut étre envisagé comme la 
premiére écaille de la ligne latérale, dont le bord est également dentelé. On 
pourrait dire aussi que le scapulaire n’est qu'une trés grande écaille de la 
partie antérieure des flancs*.” And he adds, “‘L’opinion que j’ai émise 4 
leur égard prouve que je suis loin d’admettre les rapports que l’on a cru 
trouver entre les piéces operculaires et les osselets de l’oreille interne.” 
I apprehend that the idea of the development of the opercular bones by 
the successive excretion or deposition of layers, one beneath the other, ac- 
cording to the mode in which M. Agassiz supposes scales to be formed, was 
derived merely from the appearance of the concentric lines on the opercular, 
subopercular, and interopercular bones in many fishes. I have examined 
the development of the opercular bone in young gold-fish and carp, and I 
find that it is effected in precisely the same manner as that of the frontal and 
parietal bones. The cells which regulate the intussusception and deposition 
of the earthy particles make their appearance in the primitive blastema in 
successive concentric layers, according to the same law which presides over 
the concentric arrangements of the radiated cells around the medullary canals 
in the bones of the higher vertebrata: and the term ‘successive deposition,’ 
in the sense of excretion, is inapplicable to the formation of the opercular 
bones. The argument in favour of their dermal character drawn from the 
phzenomena of the development of the opercular flap, would equally apply to 
prove the bones (ulna, radius, carpus, &c.) supporting the pectoral fin, to be 
‘dermal’ bones tf. 
The interopercular as well as the preopercular bones exist in the Lepi- 
dosiren annectens with all the characters, even to the green colour, of the rest 
of the ossified parts of the endo-skeleton ; the preopercular, as an appendage 
to the tympanic arch, retaining its primitive embryonal subcylindrical form, 
the interopercular being partly attached to the hyoid arch. Of the supra- 
scapular there is no trace in the lepidosiren; but in the sturgeon it plainly 
exists as part of the cartilaginous endo-skeleton, under the same bifurcate 
form, and double connection with the cartilaginous skull, which it presents 
in most osseous fishes. The large triangular bony dermal scale firmly adheres 
to its broad, triangular, flat, outer surface. The epi- and meso-tympanic 
cartilages in like manner expand posteriorly, and give a similar support to 
the large opercular ganoid scale. Were the supporting cartilages of the 
opercular and suprascapular scales to become ossified in the sturgeon, they 
might become anchylosed to the dermal bony plates, and bones, truly homo- 
logous with the opercular and suprascapular in ordinary osseous fishes, 
_ would thus be composed of parts of the endo- and exo-skeleton blended 
_ together. I cannot, therefore, concur with Von Baer in the opinion that the 
opercular bones are ribs of the exo-skeleton, nor with Agassiz that both the 
opercular and suprascapular bones are merely modified scales. In explaining 
my views of the opercular bones, I am compelled, believing them to have no 
special homologues in higher animals, to express those views in the terms of 
a higher generalization. The suprascapular bone (fig. 5, 40) is the upper or 
first part of the hemal arch of the occipital segment of the skull, and corre- 
sponds in serial homology with the epi-tympanic portion (2s a) of the mandi- 
_ bular arch, and with the palatine portion (20) of the maxillary arch. The 
opercular bones are the diverging appendages of the tympano-mandibular 
* Recherches sur les Poissons Fossiles, livraison 6me, 1836, tom. iv. p. 69. 
+ Ib. p. 73. 
{ “L’embryologie nous prouve, en effet, que la formation de l’appareil operculaire n’est 
bo qu’un simple produit de la peau, qui peu-a-peu s’étend par dessus les branchies, d’abord 
_ entiérement dégagées dans l’embryon.”—ZJb. p. 64. 
1846. R 
