ON THE VERTEBRATE SKELETON. 233 



l'inter-opercule. Le supra-scapulaire meme peut etre envisage comrae la 

 premiere ecaille de la ligne laterale, dont le bord est egalement dentele. On 

 pourrait dire aussi que le scapulaire n'est qu'une tres grande ecaille de la 

 partie anterieure des flancs*." And he adds, "L'opinion que j'ai emise a 

 leur egard prouve que je suis loin d'admettre les rapports que Ton a cru 

 trouver entre les pieces operculaires et les osselets de l'oreille internet." 



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 

 phaenomena 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 J. 



The interopercular as well as the preopercular bones exist in the Lejri- 

 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 haemal arch of the occipital segment of the skull, and corre- 

 sponds in serial homology with the epi-tympanic portion (28 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 



* Eecherches sur les Poissons Fossiles, livraison 6me, 1836, torn. iv. p. 69. 



t lb. p. 73. 



t " L'embryologie nous prouve, en effet, que la formation de l'appareil operculaire n'est 

 qu'un simple produit de la peau, qui peu-a-peu s'etend par dessus les brancbies, d'abord 

 entierement degagees dans l'enabrj'on." — Id. p. 64. 



1846. r 



