DERMAL BONES OF FISHES. i'69 



dire aussi que le scapulaii'e n'est qu'une tres granclc ecaillc de la 

 pai'tie antcrieure des flancs" (xxii. livraison 6me, 1836, torn. iv. 

 p. 69.). And he adds, "L'opinion que j'ai emise a leur egard prouve 

 que je suis loin d'admetti'e les rapports que Ton a cru trouver entre 

 les pieces operculaires et les osselets de I'oreille interne" (lb. p. 73.). 



I apprehend that the idea of the development of the opercular 

 bones by the successive excretion or deposition of layers, one beneath 

 the other, according to the mode in which M. Agassiz supposes scales 

 to be formed, was derived merely from the appearance of the con- 

 centric lines on the opercular, sub-opercular, and inter-opercular 

 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 pari- 

 etal bones. The cells which regulate the intus-susception and depo- 

 sition 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 Verte- 

 brata : andthetei*m " successive deposition," in the sense of excretion, 

 is inapplicable to the formation of the opercular bones. 



The inter-opercular as well as the pre-opercular bones exist in 

 the Lepidosiren annectens with all the characters, even to the green 

 colour, of the rest of the ossified parts of the endo-skeleton : the pre- 

 opercular as an appendage to the tympanic arch, the inter-opercular 

 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 

 {fig. 43. 5o) as part of the cartilaginous endo-skeleton, under the 

 same bifurcate form, and double connection with the cartilaginous 

 skull, as we have seen it to present in most Osseous Fishes. The large 

 triangular bony scale {ib. d so) firmly adheres to its broad, triangular, 

 flat, outer surface. The epi- and meso-tympanic cartilages {ib. 25, 2g) 

 in like manner expand posteriorly, and give a similar support to the 

 large opercular scale. Were the supporting cartilages of the oper- 

 cular and supra-scapular scales to become ossified in the Sturgeon, 

 they could doubtless become anchylosed to the dermal bony plates, 

 and bones, truly homologous with the opercular and supra-scapular 

 in ordinary Osseous Fishes, would thus be composed of parts of the 

 cndo- and exo-skeleton blended together. I cannot, therefore, concur 

 with Von Baer in the opinion tliat the o])ercular bones are ril)s of the 

 exo-skeleton, nor witli Agassiz that both tlie opercular and su])ra- 

 scapular bones an; merely modified scales. Tlu^ supra- scapular bone 

 is tli(! pleura[)ophysial element of tlie occipital arch, /. c. the upper or 

 first part of the ha?raal arch of that vertebra, and corresponds in 



