140 LECTUEE VI. 



serial homology with the epi-tympanic portion of the raandibular 

 arch, and with the palatine portion of the maxillary arch. The oper- 

 cular bones are the diverging appendages of the tympano-mandibular 

 arch, and correspond, in serial homology, with the branchiostegal 

 appendages of tlie hyoid and the pectoi-al appendages of the scapular 

 arches, and have the same title to be regarded as cephalic fins, and as 

 parts of the normal system of the vertebrate endo-skeleton ; but 

 neither opercuhir bones nor branchiostegal rays are retained in tlie 

 skeletons of higher Vertebrata. All diverging appendages of ver- 

 tebral segments make their first appearance in the vertebrate series 

 as 'rays ;' and the opercular bones are actually represented by car- 

 tilaginous rays, retaining tlieir primitive form in tlie Plagiostomes. 

 In the Conger the sub-opercular still presents the form of a long and 

 slender fin-ray. 



The opercular and sub-opercular may, in ordinary Osseous Fishes, 

 frequently coalesce, like the supra-scapular, with their representative 

 scales of the dermal system ; but they are essentially something more 

 than peculiarly developed representatives of those scales. M. Agassiz, 

 indeed, excepts the pre-opercular bone from the category of " pieces 

 cutanees," believing it to be the homologue of the styloid process of 

 the temporal bone in Anthropotomy, or the ' stylo-hyal' of Ver- 

 tebrate Anatomy, as the piece, viz. which completes the hyoid 

 arch above. " C'est en effet," he says, " cet os a la face interne 

 duquel I'os hyolde des poissons est suspendu, qui s'articule en haut 

 avec le mastoidien et tres souvent meme sur I'ecaille du temporal." 

 So far as my observation has gone, it is a rare exception to find the 

 hyoid arch suspended to the pre-operculum ; the rule in Osseous 

 Fishes is to find the upper styliform piece of the hyoid arch attached 

 to the epi-tympanic {mastoidien of Agassiz), close to its junction 

 with the meso-tympanic bone. It is equally the rule to find the pre- 

 opercular articulated with the epi-, meso-, and hypo-tympanics ; and 

 it is an exception, when it rises so high as to be connected with the 

 mastoid {ecaille du temporal of Agassiz). If the stylo-hyal be not 

 the upper piece of the hyoid arch displaced, and if the upper piece 

 connecting that arch with the mastoid is to be sought for in Osseous 

 Fishes, I should rather view it in the posterior half of the epi- 

 tympnnic, which is usually bifurcate below and very commonly also 

 above, when the posterior upper fork articulates with the mastoid, 

 and the posterior lower fork with the hyoid arch. 



The normal position, form, and connections of the pre-operculum 

 clearly bespeak it to be the first or proximal segment of the radiated 

 nppondage of tlic tympnno-niandilmlar arch : the opercular, sub- 

 opercular, and inter-opercular bones form the distal segment of the 



