ON TUB VERTEBRATE SKELETON. 231 



unity, whether it be expanded into a 'bulla ossea,' extended into a long tube 

 or meatus, or both, as in fig. 24, 2s, or whether, as in fig. 25, it be reduced to 

 a mere ring or hoop supporting the tympanic membrane, until it coalesces 

 with other parts of the temporal, to form the tympanic or ' external auditory 

 process' of that bone. In no air-breathing vertebrate have I ever found, or 

 seen described, the separation of the part of the tympanic forming the wall 

 of the tympanic chamber from the part supporting the tympanic membrane, 

 or this distinct, save in batrachia, from the part supporting the lower jaw*. 

 The tympanic pedicle is still further subdivided in fishes; but M. Agassiz's 

 original idea of the 'epitympanic' as a dismemberment of the pedicle, which 

 he proposed to call ' os carre superieur,' is, in my opinion, much more consist- 

 ent with nature than his later determination of that bone as the ' mastoid,' 

 or than Cuvier's attempts to find the homologues of both the mammalian 

 'squamosal' and 'jugal' in the piscine subdivisions of the same pedicle. 

 There is as little ground for making the zygomatic process a distinct element 

 from the squamous portion, as for severing the annular process from the rest 

 of the tympanic. This idea of the zygomatic as an independent piece, which 

 Dr. Kostlin has also adopted, seems to rest only on the mal-determination 

 by Bojanus and Oken of the true squamosal in birds and reptiles as the 

 'zygomaticum' or 'jugale posterius': and the idea was perhaps further 

 strengthened in the mind of M. Agassiz, by what he deems to be the essen- 

 tial and characteristic function of the squamosal. But its protective cere- 

 bral or cranial scale is a peculiarly mammalian development ; much reduced 

 in the ruminants and cetacea, and totally disappearing in the oviparous ver- 

 tebrates. The zygomatic functions and connections are, notwithstanding a 

 few exceptions, as in the scaly manis and a few lizards, the essential homo- 

 logical characters of the ' squamosal.' The necessity for forming an opinion 

 of the essential nature and general homologies of the parts blended together 

 in the human ' os temporis' by the ascensive or synthetic method, is strikingly 

 exemplified by the results of the application of M. Agassiz's idea of its nature 

 to his determination of the bones in the head of fishes. 



As the palato-maxillary arch in most air-breathing vertebrates supports, ac- 

 cording to my views, certain appendages, e. g. the malar and squamosal, which 

 are not present in fishes ; so, I believe, with Cuvier, that the tympano-man- 

 dibular arch supports in fishes, certain appendages, which are not developed 

 in any other class. It is this fact, chiefly, that has led to so much discrepancy 

 in the attempts to determine by reference to bones in higher vertebrates the 

 opercular bones of fishes, — the chief battle-field of homological controversy. 

 All the four opercuiar bones forming the diverging appendage of the tym- 

 pano-mandibular arch (fig. 5, 34 to 37) were deemed by Cuvier to be peculiar 

 ichthyic super-additions to the ordinary vertebrate skeleton ; whilst by Spix, 

 Geoffroy, and De Blainville they are held to be modifications of parts which 



* M. Agassiz applies the subjoined analysis of the 'temporal bone' to elucidate the homo- 

 logies of the skull of fishes : — " Nous distinguons encore dans le temporal complet les parties 

 suivantes : Yecaille, servant de complement a, la paroi laterale du crane dans sa partie poste- 

 rieure ; le mastoidien, servant de rempart posterieur a la cavite tympanal ; la caisse, logeant 

 les parties principales de la cavite tympanale ; I'anneau tympanique, servant d'appui a la 

 membrane du tympan ; Y apophyse jugal, formant l'appui posterieur de l'arcade zygomatique ; 

 Y apophyse stylo'ide, offrant une insertion a l'os hyoide, par laquelle ce dernier se fixe au crane ; 

 et enfin l'os carre, formant la surface articulaire sur laquelle la machoire inferieure exerce 

 ses mouvemens. La maniere variee dont ces differentes pieces se soudent ensemble, se separent 

 et se combinent, occasionnent ces innombrables variations auxquelles le temporal est sujet 

 dans son ensemble. L'eeaille du temporal est destinee, comme nous venons de le voir, a pro- 

 teger les parties cerebrales posterieures de la tete, sur la face laterale du crane." — Recherches 

 sur les Poissons Fossiles, t. ii. pt. 2, 1843, p. 62. 



