ON THE VERTEBRATE SKELETON. 931 
unity, whether it be expanded into a ‘bulla ossea, extended into a long tube 
or meatus, or both, as in fig. 24, 28, 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 carré supérieur,’ 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 squamusal, 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 opercular 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 
_ suiyantes: l’écaille, servant de complément a la paroi latérale du crane dans sa partie posté- 
rieure; le mastoidien, servant de rempart postérieur a la cavité tympanal ; Ja caisse, logeant 
les parties principales de la cavité tympanale; l’anneau tympanique, servant d’appui a la 
membrane du tympan; l’apophyse jugal, formant l’appui postérieur de l’arcade zygomatique 5 
_  Vapophyse styloide, offrant une insertion a l’os hyoide, par laquelle ce dernier se fixe au crane; 
et enfin l’os carré, formant la surface articulaire sur laquelle la machoire inférieure exerce 
ses mouvemens. . La maniére variée dont ces différentes piéces se soudent ensemble, se séparent 
et se combinent, occasionnent ces innombrables variations auxquelles le temporal est sujet 
dans son ensemble. L’écaille du temporal est destinée, comme nous venons de le voir, 4 pro- 
__ téger les parties cérébrales postérieures de la téte, sur la face latérale du crane.” —Recherches 
sur les Poissons Fossiles, t. ii. pt. 2, 1843, p. 62. 
